Adaptive data streaming method with push messages control

ABSTRACT

There is provided methods for managing streaming over communication networks. Server and client devices share a push policy so that the client device may anticipate data pushes by the server. Anticipation makes it possible to cancel early the sending of some pushed data, therefore reducing bandwidth consumption. The shared push policy may be implicit to both server and client. In embodiments, it is explicitly specified by the server to the client, for example embedded in the Media Presentation Description file or included in a specific HTTP header. The client may also request an update of the shared push policy to meet its own requirements.

This application claims priority from the following GB patentapplications: No. 1312547.1 and No. 1312561.2, both of Jul. 12, 2013,and No. 1410540.7 of Jun. 12, 2014, which are all incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to data streaming over HTTP communicationnetworks.

More particularly, the present invention relates to adaptive datastreaming for satisfying network constraints. The invention may haveapplications in DASH networks.

DASH (acronym for Dynamic Adaptive Streaming over HTTP) is acommunication standard allowing media content streaming (typicallyaudio/video content) over HTTP. According to DASH, media presentationsare described as XML files, called “media presentation description”files (MPD in what follows). MPD files provide client devices withinformation allowing them to request and control the delivery of mediacontents.

BACKGROUND OF THE INVENTION

The general principle of Media streaming over HTTP is illustrated inFIG. 3. Most of the new protocols and standards for adaptive mediastreaming over HTTP are based on this principle.

A media server 300 streams data to a client 310. The media server storesmedia presentations. For example, media presentation 301 contains audioand video data. Audio and video may be interleaved in a same file. Theway the media presentation is built is described in what follows withreference to FIG. 4a . The media presentation is temporally split intosmall independent and consecutive temporal segments 302 a, 302 b and 302c, such as MP4 segments, that can be addressed and downloadedindependently. The downloading addresses (HTTP URLs) of the mediacontent for each of these temporal segments are set by the server to theclient. Each temporal segment of the audio/video media content isassociated with one HTTP address.

The media server also stores a manifest file document 304 (described inwhat follows with reference to FIG. 5) that describes the content of themedia presentation including the media content characteristics (e.g. thetype of media: audio, video, audio-video, text etc.), the encodingformat (e.g. the bitrate, the timing information etc.), the list oftemporal media segments and associated URLs. Alternatively, the documentcontains template information that makes it possible to rebuild theexplicit list of the temporal media segments and associated URLs. Thisdocument may be written using the eXtensible Markup Language (XML).

The manifest file is sent to the client. Upon receipt of the manifestfile during a step 305, the client is informed of the associationbetween temporal segments of the media contents and HTTP addresses.Also, the manifest file provides the client with the informationconcerning the content of the media presentation (interleavedaudio/video in the present example). The information may include theresolution, the bit-rate etc.

Based on the information received, the HTTP client module 311 of clientcan emit HTTP requests 306 for downloading temporal segments of themedia content described in the manifest file. The server's HTTPresponses 307 convey the requested temporal segments. The HTTP clientmodule 311 extracts from the responses the temporal media segments andprovides them to the input buffer 307 of the media engine 312. Finally,the media segments can be decoded and displayed during respective steps308 and 309.

The media engine 312 interacts with the DASH control engine 313 in orderto have the requests for next temporal segments to be issued at theappropriate time. The next segment is identified from the manifest file.The time at which the request is issued depends on whether or not thereception buffer 307 is full. The DASH control engine 313 controls thebuffer in order to prevent it from being overloaded or completely empty.

The generation of the media presentation and the manifest file isdescribed with reference to FIG. 4a . During steps 400 and 401, audioand video data are acquired. Next, the audio data are compressed during402. For example, the MP3 standard can be used. Also, the video data arecompressed in parallel during step 403. Video compression algorithmssuch as MPEG4, MPEG/AVC, SVC, HEVC or scalable HEVC can be used. Oncecompression of audio and video data has been performed, audio and videoelementary streams 404, 405 are available. The elementary streams areencapsulated during a step 406 into a global media presentation. Forexample, the ISO BMFF standard (or the extension of the ISO BMFFstandard to AVC, SVC, HEVC, scalable extension of HEVC etc.) can be usedfor describing the content of the encoded audio and video elementarystreams as a global media presentation. The encapsulated mediapresentation 407 thereby obtained is used for generating, during step408, an XML manifest file 409. Several representations of video data 401and audio data 400 can be acquired, compressed, encapsulated anddescribed in the media presentation 407.

For the specific case of MPEG/DASH streaming protocol illustrated inFIG. 4b , the manifest file is called “Media Presentation Description”(or “MPD” file). The root element of the file is the MPD element thatcontains attributes applying to all the presentation plus DASHinformation like profile or schema. The media presentation is split intotemporal periods represented by a Period element. The MPD file 410contains all the data related to each temporal period. By receiving thisinformation, the client is aware of the content for each period of time.For each Period 411, AdaptationSet elements are defined.

A possible organization is to have one or more AdaptationSet per mediatype contained in the presentation. An AdaptationSet 412 related tovideo contains information about the different possible representationsof the encoded videos available at the server. Each representation isdescribed in a Representation element. For example, a firstrepresentation can be a video encoded with a spatial resolution of640×480 and compressed with a bit rate of 500 kbits/s. A secondrepresentation can be the same video but compressed with a bit rate of250 kbits/s.

Each video can then be downloaded by HTTP requests if the client knowsthe HTTP addresses related to the video. The association between thecontent of each representation and the HTTP addresses is done by usingan additional level of description: the temporal segments. Each videorepresentation is split into temporal segments 413 (typically fewseconds). Each temporal segment comprises content stored at the serverthat is accessible via an HTTP address (URL or URL with one byte range).Several elements can be used for describing the temporal segments in theMPD file: SegmentList, SegmentBase or Segment Template.

In addition, a specific segment is available: the initializationsegment. The initialization segment contains MP4 initializationinformation (if the video has been encapsulated using the ISO BMFF orextensions thereof) that describes the encapsulated video stream. Forexample, it helps the client to instantiate the decoding algorithmsrelated to the video.

The HTTP addresses of the initialization segment and the media segmentsare indicated in the MPD file.

In FIG. 5, there is shown an exemplary MPD file. Two media are describedin the MPD file shown. The first one is an English audio stream and thesecond one is a video stream. The English audio stream is introducedusing the AdaptationSet tag 500. Two alternative representations areavailable for this audio stream:

-   -   the first representation 501 is an MP4 encapsulated elementary        audio stream with a bit-rate of 64000 bits/sec. The codec to be        used for handling this elementary stream (after MP4 parsing) is        defined in the standard by the attribute codecs having the        value: ‘mp4a.0x40’. It is accessible via a request at the        address formed by the concatenation of the BaseURL elements in        the segment hierarchy: <BaseURL>7657412348.mp4</BaseURL>, which        is a relative URI. The <BaseURL> being defined at the top level        in the MPD element by ‘http://cdn1.example.com/’ or by        ‘http://cdn2.example.com/’ (two servers are available for        streaming the same content) is the absolute URI. The client can        then request the English audio stream from the request to the        address ‘http://cdn1.example.com/7657412348.mp4’ or to the        address ‘http://cdn2.example.com/7657412348.mp4’.    -   the second representation 502 is an MP4 encapsulated elementary        audio stream with a bit-rate of 32000 bits/sec. The same        explanations as for the first representation 501 can be made and        the client device can thus request this second representation        502 by a request at either one of the following addresses:

‘http://cdn1.example.com/3463646346.mp4’ or

‘http://cdn2.example.com/3463646346.mp4’.

The adaptation set 503 related to the video contains sixrepresentations. These representations contain videos with differentspatial resolutions (320×240, 640×480, 1280×720) and with different bitrates (from 256000 to 2048000 bits per second). For each of theserepresentations, a respective URL is associated through a BaseURLelement. The client can therefore choose between these alternativerepresentations of the same video according to different criteria likeestimated bandwidth, screen resolution etc. (Note that, in FIG. 5, thedecomposition of the Representation into temporal segments is notillustrated, for the sake of clarity.)

FIG. 5a shows the standard behavior of a DASH client. FIG. 5b shows atree representation of an exemplary manifest file (description file orMPD) used in the method shown in FIG. 4 a.

When starting a streaming session, a DASH client starts by requestingthe manifest file (step 550). After waiting for the server's responseand receiving the manifest file (step 551), the client analyzes themanifest file (step 552), selects a set AS_(ij) of AdaptationSetssuitable for its environment (step 553), then selects, within eachAdaptationSet AS_(ij), a Representation in the MPD suitable for examplefor its bandwidth, decoding and rendering capabilities (step 554).

The DASH client can then build in advance the list of segments torequest, starting with initialization information for the mediadecoders. This initialization segment has to be identified in the MPD(step 555) since it can be common to multiple representations,adaptation sets and periods or specific to each Representation or evencontained in the first media segment.

The client then requests the initialization segment (step 556). Once theinitialization segment is received (step 557), the decoders getinitiated (step 558).

The client then requests first media data on a segment basis (step 560)and buffers a minimum data amount (thanks to the condition at step 559)before actually starting decoding and displaying (step 563). Thesemultiple requests/responses between the MPD download and the firstdisplayed frames introduce a startup delay in the streaming session.After these initial steps, the DASH streaming session continues in astandard way, i.e. the DASH client adapts and requests the mediasegments one after the other.

The current DASH version does not provide description ofRegion-Of-Interest within the manifest files. Several approaches havebeen proposed for such description.

In particular, components of media contents can be described usingSubRepresentation elements. These elements describe the properties ofone or several components that are embedded in a Representation. In FIG.6, there is shown an example of a DASH manifest file describing tiletracks as components of a video. For the sake of conciseness andclarity, only one Period 600 is represented. However, subsequent periodelements would be organized in a same fashion. In part 601, a firstadaptation set element is used for describing a base layer of thescalable video. For example, the video is encoded according to SVC orHEVC scalable. In part 602, a second adaptation set is used fordescribing the highest resolution layer of the scalable video. Fornon-scalable video, only the second adaptation set 602 would be present,without dependency to the base layer, i.e. the dependencyId attribute.In this second adaptation set 602, a single representation 603 isdescribed, namely the one that corresponds to the displayable video. Therepresentation is described as a list of segments 610 with respectiveURLs for client requests.

Thus, the representation depends on another representation identified by‘R1’ (dependencyId attribute), actually the base layer representationfrom the first adaptation set 601. The dependency forces the streamingclient to first request the current segment for base layer beforegetting the current segment for the enhancement layer. This cannot beused to express dependencies with respect to tile tracks because thetracks that would be referenced this way would be automatically loadedby the client. This is something to be avoided, since it is up to theuser to select the tiles of interest for him anytime during the mediapresentation. Therefore, in order to indicate the dependencies betweenthe composite track and the tile tracks the SubRepresentation element isused. The displayable video is described as a list ofsub-representations 604 to 608. Each sub representation actuallyrepresents a track in the encapsulated MP4 file. Thus, there is onesub-representation per tile (four tiles in the present example) plus onesub-representation for the composite track 608. Each sub-representationis described by a content component element 614 to 618 in order toindicate whether it corresponds to a tile track 614, 615, 616 and 617 orto the composite track 618. The Role descriptor type available inDASH/MPD is used with a specific scheme for tiling. The Role descriptoralso indicates the position of the tile in the full-frame video. Forexample the component 614 describes the tile located at the top left ofthe video (1:1 for first in row and first in column). The dimensions ofthe tiles, width and height, are specified as attributes of the subrepresentation as made possible by MPD. Bandwidth information can alsobe put here for helping the DASH client in the determination of thenumber of tiles and the selection of the tiles, according to itsbandwidth. Concerning the composite track, it has to be signalled in adifferent way than the tile tracks since it is mandatory to be able, atthe end of the download, to build a video stream that can be decoded. Tothat purpose, two elements are added into the description. Firstly, thedescriptor in the related content component 618 indicates that it is themain component among all the components. Secondly, in the subrepresentation, a new attribute ‘required’ is added in order to indicateto the client that the corresponding data have to be requested. Allrequests for the composite track or for one or more of the tile tracksare computed from the URL provided in the segment list 610 (one per timeinterval). In the example, “URL_X” combined with “BaseURL” at thebeginning of the MPD provides a complete URL which the client can usefor performing an HTTP GET request. With this request, the client wouldget the data for the composite track and all the data for all the tiletracks. In order to optimize the transmission, instead of the request,the client can first request the segment index information (typicallythe “ssix” and/or “sidx” information in ISO BMFF well known by the manskilled in the art), using the data available from the index_rangeattribute 620. This index information makes it possible to determine thebyte ranges for each of the component. The DASH client can then send asmany HTTP GET requests with appropriate byte range as selected tracks(including the required composite track).

When starting a streaming session, a DASH client requests the manifestfile. Once received, the client analyzes the manifest file, selects aset of AdaptationSets suitable for its environment. Next, the clientselects in the MPD, within each AdaptationSet, a Representationcompatible with its bandwidth, decoding and rendering capabilities.Next, it builds in advance the list of segments to be requested,starting with initialization information for the media decoders. Wheninitialization information is received by the decoders, they areinitialized and the client requests first media data and buffers aminimum data amount before actually starting the display.

These multiple requests/responses may introduce delay in the startup ofthe streaming session. The risk is for service providers to see theirclients leaving the service without starting to watch the video. It iscommon to name this time between the initial HTTP request for the firstmedia data chunk, performed by the client, and the time when the mediadata chunk actually starts playing as the start-up delay. It depends onthe network round-trip time but also on the size of the media segments.

Server Push is a useful feature for decreasing web resource loadingtime. Such servers are discussed with reference to FIGS. 1a to 1 e.

In FIG. 1b , there is shown that in HTTP/2 exchanges, a request must besent for every resource needed: resources R1 to R4 and sub-resources Ato I (as shown in FIG. 1a ). However, when using the push feature byservers, as illustrated in FIG. 1c , the number of requests is limitedto elements R1 to R4. Elements A to I are “pushed” by the server to theclient based on the dependencies shown in FIG. 1a , thereby making theassociated requests unnecessary.

Thus, as illustrated in FIGS. 1b and 1c , when servers use the pushfeature, the number of HTTP round-trips (request+response) necessary forloading a resource with its sub-resources is reduced. This isparticularly interesting for high-latency networks such as mobilenetworks.

HTTP is the protocol used for sending web resources, typically webpages. HTTP implies a client and a server:

-   -   The client sends a request to the server;    -   The server replies to the client's request with a response that        contains a representation of the web resource.

Requests and responses are messages comprising various parts, notablythe HTTP headers. An HTTP header comprises a name along with a value.For instance, “Host: en.wikipedia.org” is the “Host” header, and itsvalue is “en.wikipedia.org”. It is used for indicating the host of theresource queried (for instance, the Wikipedia page describing HTTP isavailable at http://en.wikipedia.org/wiki/HTTP). HTTP headers appear onclient requests and server responses.

HTTP/2 makes it possible to exchange requests/responses through streams.A stream is created inside an HTTP/2 connection for every HTTP requestand response. Frames are exchanged within a stream in order to conveythe content and headers of the requests and responses.

HTTP/2 defines a limited set of frames with different meanings, such as:

-   -   HEADERS: which is provided for transmission of HTTP headers    -   DATA: which is provided for transmission of HTTP message content    -   PUSH_PROMISE: which is provided for announcing pushed content    -   PRIORITY: which is provided for setting the priority of a stream    -   WINDOW_UPDATE: which is provided for updating the value of the        control flow window    -   SETTINGS: which is provided for conveying configuration        parameters    -   CONTINUATION: which is provided for continuing a sequence of        header block fragments    -   RST_STREAM: which is provided for terminating or cancelling a        stream.

Push by servers has been introduced in HTTP/2 for allowing servers tosend unsolicited web resource representations to clients. Web resourcessuch as web pages generally contain links to other resources, whichthemselves may contain links to other resources. To fully display a webpage, all the linked and sub-linked resources generally need to beretrieved by a client. This incremental discovery may lead to a slowdisplay of a web page, especially on high latency networks such asmobile networks.

When receiving a request for a given web page, the server may know whichother resources are needed for the full processing of the requestedresource. By sending the requested resource and the linked resources atthe same time, the server allows reducing the load time of the web page.Thus, using the push feature, a server may send additional resourcerepresentations at the time it is requested a given resource.

With reference to the flowchart of FIG. 1e , an exemplary mode ofoperation of a server implementing the push feature is described.

During step 100, the server receives an initial request. Next, theserver identifies during step 101 the resources to push as part of theresponse and starts sending the content response during step 102. Inparallel, the server sends push promise messages to the client duringstep 103. These messages identify the other resources that the server isplanning to push, for instance based on the dependencies shown in FIG.1a . These messages are sent in order to let the client know in advancewhich pushed resources will be sent. In particular, this reduces therisk that a client sends a request for a resource that is being pushedat the same time or about to be pushed. In order to further reduce thisrisk, a server should send a push promise message before sending anypart of the response referring to the resource described in the pushpromise. This also allows clients to request cancellation of the push ofthe promised resources if clients do not want those resources. Next, theserver sends the response and all promised resources during step 104.The process ends during a step 105.

The flowchart of FIG. 1d illustrates the process on the client side.

When the client has identified a resource to retrieve from the server,it first checks during a step 106 whether or not the corresponding datais already in its cache memory. In case the resource is already in thecache memory (Yes), it is retrieved from it during a step 107. Cacheddata may be either data retrieved from previous requests or data thatwere pushed by the server previously. In case it is not in the cachememory (No), the client sends a request during step 108 and waits forthe server's response. Upon receipt of a frame from the server, theclient checks during step 109 whether or not the frame corresponds to aPUSH promise. If the data frame corresponds to the PUSH promise (Yes),during step 110, the client processes the push promise. The clientidentifies the resource to be pushed. If the client does not wish toreceive the resource, the client may send an error message to the serverso the server does not push that resource. Otherwise, the client storesthe push promise until receiving the corresponding push content. Thepush promise is used so that the client does not request the promisedresource while the server is pushing it. In case the data frame does notcorrespond to the PUSH promise (No), it is checked, during step 111,whether or not, the frame is a data frame related to push data. In caseit is related to push data (Yes), the client processes the pushed dataduring step 112. The pushed data is stored within the client cache. Incase the frame is not a data frame related to push data (No), it ischecked, during step 113, whether it corresponds to a response receivedfrom the server. In case the frame corresponds to a response from theserver (Yes), the response is processed during step 114 (e.g. sent tothe application). Otherwise (No), it is checked during step 115 whetheror not the frame identifies the end of a response (Yes). In this case,the process is terminated during step 116. Otherwise, the process goesback to step 109.

Thus, it appears that the client receives the response and the promisedresources. The promised resources are therefore generally stored in theclient cache while the response is used by the application such as abrowser displaying a retrieved web page. When a client applicationrequests one of the resources that were pushed, the resource isimmediately retrieved from the client cache, without incurring anynetwork delay.

The storage of pushed resources in the cache is controlled using thecache control directives. The cache control directives are also used forcontrolling of the responses. These directives are in particularapplicable to proxies: any resource pushed or not, may be stored byproxies or by the client only.

FIG. 1a is a graph of a set of resources owned by a server with theirrelationships. The set of resources is intertwined: R₁, R₂, R₃, and R₄are resources that need to be downloaded together to be properlyprocessed by a client. In addition, sub-resources A to H are defined.These sub-resources are related to 1, 2 or 3 resources. For instance, Ais linked to R₁ and C is linked to R₁, R₂ and R₄.

FIG. 1b , already discussed hereinabove, shows an HTTP exchange withoutusing the server PUSH feature: the client requests R₁, next it discoversR₂, A, B, C and D and request them. After receiving them, the clientrequests R₃, R₄, F and G. Finally the client requests H and Isub-resources. This requires four round-trips to retrieve the whole setof resources.

FIG. 1c , already discussed hereinabove, illustrates the HTTP exchangeusing the feature of pushing directly connected sub-resources by theserver. After requesting R₁, the server sends R₁ and pushes A, B, C andD. The client identifies R₂ and requests it. The server sends R₂ andpushes F and G. Finally the client identifies R₃, R₄ and requests theseresources. The server sends R₃, R₄ and pushes H and I. This processrequires three round-trips to retrieve the whole set of resources.

In order to decrease the loading time of a set of resources, typically aweb page and its sub-resources, HTTP/2 allows exchanging multiplerequest and response priorities in parallel. As illustrated in FIG. 2, aweb page may require the download of several resources, like JavaScript,images etc. During an initial HTTP exchange 200, the client retrieves anHTML file. This HTML file contains links to two JavaScript files (JS1,JS2), two images (IMG1, IMG2), one CSS file and one HTML file. During anexchange 201, the client sends a request for each file. The order givenin the exchange 201 of FIG. 2 is based on the web page order: the clientsends a request as soon as a link is found. The server then receivesrequests for JS1, CSS, IMG1, HTML, IMG2 and JS2 and processes theserequests according that order. The client then retrieves these resourcesin that order.

HTTP priorities make it possible for the client to state which requestsare more important and should be treated sooner than other requests. Aparticular use of priorities is illustrated in exchange 202. JavaScriptfiles are assigned the highest priority. CSS and HTML files are assignedmedium priority and images are assigned low priority. This approachallows receiving blocking files or files that may contain references toother resources sooner than other files. In response, the server isexpected to try sending sooner the JavaScript files, the CSS and HTMLfiles afterwards and the images at the end, as described in exchange202. Servers are not mandated to follow client priorities.

In addition to priorities, HTTP/2 provides that the amount of data beingexchanged simultaneously can be controlled. Client and server canspecify which amount of data they can buffer on a per connection basisand a per stream basis. This is similar to TCP congestion control: awindow size, which specifies an available buffer size, is initialized toa given value; each time the emitter sends data, the window size isdecremented; the emitter must stop sending data so that the window sizenever goes below zero. The receiver receives the data and sends messagesto acknowledge that the data was received and removed from the buffer;the message contains the amount of data that was removed from thebuffer; the window size is then increased from the given value and theemitter can restart sending data.

In view of the above, it appears that DASH is based on the assumptionthat the client leads the streaming since the client can generallyselect the best representation of the content for the purpose of theapplication it is performing. For instance, a client may know whether torequest High-Definition or Small-Definition content based on itsform-factor and screen resolution.

Server-based streaming is typically done using RTP. Contrary to DASH,RTP does not use HTTP and cannot directly benefit from the webinfrastructures, in particular proxies and caches. Web socket basedmedia streaming has the same drawbacks. With HTTP/1.1, server-basedstreaming cannot be easily implemented since the server can generallyonly answer to client requests. With HTTP/2, in particular with theintroduction of the push feature, DASH-based servers can lead thestreaming. Thus, servers can use their knowledge of the characteristicsof the content they are streaming for optimizing the user experience.For instance, a server may push a film as SD (due to limited bandwidth)but advertisements as HD since advertisements take an additional limitedamount of bandwidth. Another example is the case of a server that startsto do fast start with a low-resolution video and switches to the bestpossible representation once bandwidth is well estimated.

In order to enable a server to lead the streaming, one approach is tolet the server push data (in particular DASH data) as preferred. Theclient then uses whatever data is available to display the video. Theserver typically announces the push of several segments at once. Theserver then sends the segments in parallel or successively.

A problem that occurs is that client and server may not know if thepromised data will be transmitted and received at the desired time: theclient may not know when and in which order the video segments will besent.

Also, the promised data pushed or announced by the server may mismatchthe client needs, thus leading to resource wasting in particular at theserver end.

Thus, there is a need for enhancing data streaming especially in thecontext of DASH-based communications.

SUMMARY OF THE INVENTION

The present invention lies within this context.

According to a first aspect of the invention corresponding to theserver's perspective, a method of streaming media data by a serverdevice to a client device, comprises the following steps:

-   -   receiving, from the client device, a request relating to first        media data,    -   identifying second media data to be sent to the client device        without having been requested, and    -   in response to said request, transmitting to said client device,        data relating to said first media data, and preparing at least        one announcement message respectively identifying said second        media data with a view to transmit the announcement message or        messages to the client device, and

wherein the method further comprises the step of using a push policyshared with the client device for the server device to drive theidentification or the transmission of the second non-requested mediadata to the client device.

According to a second aspect of the invention corresponding to theclient's perspective, a method of accessing by a client device, mediadata streamed by a server device, the method comprises the followingsteps:

-   -   transmitting, to the server device, a request relating to first        media data,    -   receiving from said server device, in response to said request,        data relating to said first media data,

wherein the method further comprises the step of using a push policyshared with the server device for the client device to determine secondmedia data to be sent by the server device without having been requestedby the client device or to determine an order of transmission thereof bythe server device.

In particular, the shared push policy may define how to determine secondmedia data, for the devices to determine the second non-requested mediadata to be sent by the server device to the client device.

Thanks to this approach, the mismatch between the server's decisionregarding the media data to be pushed and the client's needs can bedecreased and thus resources can be saved.

This is achieved by using a shared push policy that makes it possiblefor the client to anticipate the server's behaviour, and thus the secondmedia data that are about to be pushed. Thanks to the shared push policythan may be used for several client's subsequent requests, the clientcan anticipate the server's behaviour even before the requests are sentto the server.

As a consequence of the anticipation, the client can prepare and requestthe cancellation of such second media data that are not needed, in ananticipated fashion with respect to the announcement by the server.

The request relating to first media data may concern first media dataand/or other data related to this first media data.

The second media data may be associated with said first media data, forexample by the server device.

Embodiments of the invention provide a lightweight mechanism forserver-guided streaming. Embodiments may be implemented in the contextof DASH networks.

Server devices can make content recommendations to the client devices.Also, they can optimize the network usage.

Embodiments of the invention are compatible with existing HTTP/2features. These features can advantageously be used for implementingembodiments of the invention.

Network performances are generally increased.

Correspondingly, the invention also regards a server device forstreaming media data to a client device, the device comprising:

-   -   a receiver configured for receiving, from the client device, a        request relating to first media data,    -   a control unit configured for identifying second media data to        be sent to the client device without having been requested, and    -   a transmitter configured for, in response to said request,        transmitting to said client device, data relating to said first        media data, and preparing at least one announcement message        respectively identifying said second media data with a view to        transmit the announcement message or messages to the client        device, and

wherein the control unit is further configured for using a push policyshared with the client device in order to drive the identification orthe transmission of the second non-requested media data to the clientdevice.

The invention also regards a client device for accessing media datastreamed by a server device, the device comprising:

-   -   a transmitter configured for transmitting, to the server device,        a request relating to first media data, and    -   a receiver configured for receiving from said server device, in        response to said request, data relating to said first media        data,

wherein the client device is configured for using a push policy sharedwith the server device in order to determine second media data to besent by the server device without having been requested by the clientdevice or to determine an order of transmission thereof by the serverdevice.

The server and client devices have the same advantages as thecorresponding methods as described above.

Optional features of the methods and devices are defined in thedependent claims. Some of them are explained below with respect to themethods. However they can also apply to the corresponding device.

In some embodiments referred below as to an explicit approach, themethod from the server's perspective further comprises:

determining by the server device a push policy, and

transmitting, from the server device to the client device, push policyinformation describing the determined push policy to share the pushpolicy with the client device.

Correspondingly at the client side, the method may further comprisereceiving, from the server device, push policy information describingthe shared push policy.

As described in some examples below, the push policy informationdescribing the shared push policy is inserted in a description file thatis transmitted from the server device to the client device, thedescription file containing description information that concerns mediadata including the first media data, the method further comprisingdetermining the second non-requested media data based on saiddescription file using the shared push policy.

In a specific embodiment, the description file describes the media datausing a plurality of media data attribute levels, and various sharedpush policies are defined at various respective levels of thedescription file.

In other examples, the push policy information describing the sharedpush policy is embedded in a header of an HTTP frame transmitted fromthe server device to the client device.

According to specific features, the method may further comprise, at theserver device, receiving push policy update information embedded in aheader of an HTTP frame from the client device, and updating accordinglythe shared push policy before determining non-requested media data fromother media data requested by the client device.

Correspondingly, the method may further comprise, at the client device,sending push policy update information embedded in a header of an HTTPframe to the server device.

According to a hybrid approach, the push policy information describingthe shared push policy is defined by a first push policy part and asecond push policy part,

the first push policy part being inserted in a description file that istransmitted from the server device to the client device, the descriptionfile containing description information that concerns media dataincluding the first media data, the method further comprisingdetermining the second non-requested media data based on saiddescription file using the shared push policy,

and the second push policy part being embedded in a header of an HTTPframe transmitted from the server device to the client device.

For example, the second push policy part may comprise one or more valuesfor one or more associated variables defined in the first push policypart.

Also, the description file may include the description of a plurality ofcandidate push policies, and the second push policy part may thuscomprise an identifier of a candidate push policy from said plurality,which identified candidate push policy thereby forming the first pushpolicy part.

In other embodiments, the push policy information includes a JavaScriptprogram embedded in a web page transmitted from the server device to theclient device.

In yet other embodiments, the method further comprises determining thesecond non-requested media data based on a structured document (such asa description file described above or an HTML page introduced inexamples below), the structured document containing descriptioninformation that concerns media data including the first media data, and

the push policy information includes an XPath expression to be evaluatedon a tree representation of the structured document to identify thesecond non-requested media data.

Regarding the syntax of the push policy information, embodiments providethat the push policy information includes a first push attributedefining an amount of second non-requested media data to be identifiedin a description file,

the description file containing description information that concernsmedia data including the first media data, and the method furthercomprising determining the second non-requested media data based on saiddescription file using the shared push policy.

According to specific features, the first push attribute identifies thesecond non-requested media data relatively to the first media datarequested within the description file. This may be done using operatorsas described below.

In a variant, the first push attribute is an identifier of specificmedia data within the description file.

According to specific features, the description information in thedescription file describes media data according to at least one mediadata attribute from amongst a period attribute defining a time period towhich the media data belong, an adaptation attribute defining a mediatype of the media data, a representation attribute defining an encodingversion (e.g. bitrate, frame rate, frame resolution, timing information,etc.) of the media data and a segment attribute defining, and

the push policy information includes at least a second push attributedefining a constraint on the media data attribute or attributes, foridentifying the second non-requested media data.

This makes it possible to have very selective push policies throughoutthe description file.

In particular, the push attribute or attributes may define a media dataattribute or attributes of the second non-requested media datarelatively to the corresponding media data attribute or attributes ofthe first media data within the description file.

Alternatively, the push attribute or attributes may identify a node inthe description file in which the second non-requested media data haveto be retrieved.

In some embodiments, the description information in the description fileincludes priority attributes associated with the media data, onepriority attribute for each media data, and an order of transmission ofthe second media data is based on the associated priority attributes.This is to define an order of transmission of the push data.

In embodiments, the shared push policy identifies the second media datafrom the first media data requested.

In embodiments referred below as to an implicit approach, the sharedpush policy is implemented using the same second media data determiningalgorithm at both the server device and the client device, the algorithmenabling the server device and the client device to determine the samesecond media data from the first media data requested.

In some embodiments adapted to both the implicit and explicitapproaches, if the identified second media data comprise a plurality ofmedia segments, each requiring an announcement message, the method mayfurther comprise merging the corresponding plurality of announcementmessages into a single announcement message to be transmitted to theclient device. This is to reduce bandwidth consumption since lessannouncement messages will be sent.

To take actual advantage of the shared push policy and consequentanticipation of pushes by the client device, the method may furthercomprise receiving, from the client device, a cancellation requestrequesting to cancel transmission of a part of the second non-requestedmedia data so that the server device does not transmit the correspondingprepared announcement message.

Correspondingly at the client, the method may further comprise sending,to the server device, a cancellation request requesting to canceltransmission of a part of the second non-requested media data, in orderto drive the server device not to transmit an announcement messageidentifying the part of the second non-requested media data.

In embodiments of the invention, the second non-requested media data aredetermined by the client device independently of at least oneannouncement message prepared by (and possibly received from) the serverdevice and identifying the second non-requested media data the serverdevice intends to send to the client device without having beenrequested. Here, “independently” means that the client device is able tomake the determination of the second non-requested data without beingaware of such announcement message (i.e. PUSH_PROMISE) which isdedicated to inform the client device of the future transmission of suchnon-requested data.

In other embodiments of the invention, the same shared push policy isused for determining respective non-requested media data from aplurality of requests relating to respective first media data. By usingthe same push policy over time and successive requests, the client iseven more in good position to efficiently anticipate the transmission ofuseless data by the server, and thus in position to efficiently canceltheir transmission and the transmission of corresponding announcementmessages.

Regarding the notification of an order of transmission of the push datafrom the server to the client, a method of streaming media data by aserver device to a client device, may comprise the following steps:

-   -   receiving, from the client device, a request relating to first        media data,    -   identifying second media data to be sent to the client device        without having been requested,    -   transmitting to said client device, in response to said request,        data relating to said first media data, and at least one        announcement message respectively identifying said second media        data, and

wherein the method further comprises the following steps:

-   -   defining by the server device an order of transmission of the        second media data (this form all or part of the shared push        policy),    -   transmitting information related to the order of transmission        with said announcement messages, said information enabling the        client device to determine the order of transmission defined by        the server.

For example, the order of transmission of said second media is definedaccording to priority values according to the client device, the mediadata having the highest priority value being transmitted first.

Said priority values may be defined according to the HTTP/2 protocol.

According to embodiments, at least one priority value is associated witha network bandwidth estimation mechanism, and the method furthercomprises the following steps:

-   -   transmitting to the client device second media data with a        priority value associated with said mechanism,    -   receiving from the client device, in response to said second        media data, at least one control flow message, and    -   estimating an available bandwidth based on said at least one        control flow message received.

For example, the server device transmits said second media dataaccording to a plurality of data frames having respective and differentsizes.

The method may further comprise defining by the server device, based onsaid bandwidth estimation, an updated order of transmission of thesecond media data.

According to embodiments said request from the client device comprises arequest for receiving a description file related to media datacomprising said first media data, the description file containingdescription information concerning said first media data, the methodfurther comprising determining the second non-requested media data basedon said description file.

For example, requested first media data are video segments.

The streaming may be performed according to the DASH standard.

For example, the method further comprises the following steps:

-   -   receiving, from the client device, an ordering update request,    -   defining, based on said ordering update request, a new order of        transmission of the second media data and updating the        information related to said new order of transmission of the        second media data, and    -   transmitting said second media data to the client according to        said updated information related to the order of transmission.

The method may further comprise transmitting to the client device, anordering update confirmation message.

For example, said updated order is defined for the second media data forwhich transmission to the client device has not been initiated at thetime of receipt of said ordering update request.

For example, said ordering update request comprises an ordering valuefor at least part of second media data.

According to embodiments, the order of transmission of said second mediais defined according to priority values, and when a priority value isupdated for at least part of a first media data, the priority values forat least part of second media data to be sent to the client devicewithout having been requested and associated with said at least part offirst media data, are updated accordingly.

For example, said first and second media are associated according to atleast one of a temporal relationship, a spatial relationship and aquality relationship.

According to embodiments:

-   -   said second media data comprises enhancement data for enhancing        quality of the first media data, and    -   when a priority value is updated for a media data of an        enhancement layer, priority values are updated for all the media        data of said enhancement layer.

For example, the first and second media data comprise video temporalsegments, and the starting time of the enhancement media data is basedon information related to a video content of the first media data.

For example, said information related to a video content of the firstmedia data is stored in said description file.

For example, said order of transmission is based at least on decodingrelationships between first a second media data.

For example, said order of transmission is based at least on statisticalpopularities of the media data.

For example, said order of transmission is based at least on a playingtime of the media data on the client device's end.

For example, said order of transmission is based at least on anestimated transmission time of the media data.

For example, said order of transmission is based at least onuser-defined interests for the media data.

The method may further comprise the following steps:

-   -   receiving, from the client device, control messages, said        control messages enabling the server device to identify media        data currently being played,    -   defining by the server, based on said control messages, an        updated order of transmission of the second media data, and    -   transmitting said second media data to the client according to        said updated order of transmission.

The method may further comprise a step of transmitting to the clientdevice, an ordering update confirmation message.

For example, said control messages relate to a use of a buffer memory ofthe client device, said buffer memory storing media data for them to beplayed by the client.

For example, the server device keeps record of first requested mediadata sent, and identification of the second media data is performedbased on said use of the buffer memory and said record.

For example, said order of transmission information is transmittedwithin said announcement messages.

For example, said order of transmission information is transmittedwithin dedicated messages after said announcement messages.

From the client's perspective, a method of accessing by a client device,media data streamed by a server device, may comprise the followingsteps:

-   -   transmitting, to the server device, a request relating to first        media data,    -   receiving from said server device, in response to said request,        data relating to said first media data, and at least one        announcement message respectively identifying second media to be        sent to the client device without having been requested, wherein        the method further comprises the following step:    -   receiving information related to an order of transmission of the        second media data with said announcement messages, said        information (i.e. the shared push policy) enabling the client        device to determine an order of transmission of the second media        data defined by the server.

The method may further comprise determining by the client device whetherthe order of transmission of the second media data defined by the serverdevice satisfies streaming constraints at the client device's end, andif said constraints are not satisfied, transmitting, to the serverdevice, an ordering update request.

For example, the order of transmission of said second media data isdefined according to priority values according to the client device, themedia data having the highest priority value being transmitted first.

For example, said priority values are defined according to the HTTP/2protocol.

According to embodiments, at least one priority value is associated witha network bandwidth estimation mechanism, the method further comprisesthe following steps:

-   -   receiving from the server device second media data with a        priority value associated with said mechanism,    -   transmitting to said server device, in response to said second        media data, at least one control flow message, thereby enabling        the server device to estimate an available bandwidth based on        said at least one control flow message transmitted.

For example, the client device receives said second media data accordingto a plurality of data frames having respective and different sizes.

For example, an updated order of transmission of the second media datais defined, by the server device, based on said bandwidth estimation.

For example, said request from the client device comprises a request forreceiving a description file related to media data comprising said firstmedia data, the description file containing description informationconcerning said first media data, the method further comprisingdetermining the second non-requested media data based on saiddescription file.

For example, requested first media data are video segments.

For example, said streaming is performed according to the DASH standard.

The method may further comprise receiving said second media data fromthe server device according to updated information related to a neworder of transmission of the second media data defined by the serverdevice.

The method may further comprise a step of receiving from the serverdevice, an ordering update confirmation message.

According to embodiments, said updated order is defined for the secondmedia data for which transmission from the server device has not beeninitiated at the time of receipt of said ordering update request by theserver device.

According to embodiments, said ordering update request comprises anordering value for at least part of the second media data.

According to embodiments, the order of transmission of said second mediais defined according to priority values, and when a priority value isupdated for at least part of a first media data, the priority values forat least part of second media data to be sent to the client devicewithout having been requested and associated with said at least part offirst media data, are updated accordingly.

For example, said first and second media data are related according toat least one of a temporal relationship, a spatial relationship and aquality relationship.

According to embodiments:

-   -   said second media data comprise enhancement data for enhancing        quality of the first media data, and    -   when a priority value is updated for at least part of first        media data of an enhancement layer, priority values are updated        for all the media data of said enhancement layer.

For example, the first and second media data comprise video temporalsegments, and the starting time of the enhancement media data is basedon information related to a video content of the first media data.

According to embodiments, said information related to a video content ofthe first media data is stored in said description file.

According to embodiments, said order of transmission is based at leaston decoding relationships between first and second media data.

According to embodiments, said order of transmission is based at leaston statistical popularities of the media data.

According to embodiments, said order of transmission is based at leaston a playing time of the media data on the client device's end.

According to embodiments, said order of transmission is based at leaston an estimated transmission time of the media data.

According to embodiments, said order of transmission is based at leaston user-defined interests for the media data.

The method may comprise the following steps:

-   -   transmitting, to the server device, control messages, said        control message enabling the server device to identify a media        data currently being played, and    -   receiving said second media data from the server device        according to an updated order of transmission defined, by the        server device, based on said control messages.

The method may comprise a step of receiving from the server device, anordering update confirmation message.

For example, said control messages relate to a use of a buffer memory ofthe client device, said buffer memory storing media data for them to beplayed by the client device.

According to embodiments, the server device keeps record of first mediadata sent, and identification of the media being currently played isperformed based on said use of the buffer memory and said record.

For example, said order of transmission information is received withinsaid announcement messages.

For example, said order of transmission information is received withindedicated messages after said announcement messages.

Still referring to the order of transmission, a method of managing, by aproxy server, data exchanges between client devices and server devices,may comprise the following steps:

-   -   receiving, from a server implementing a method as defined above        regarding the notification of an order of transmission, media        data to be retransmitted to a client device,    -   determining, based on the order of transmission of the media        data, a retransmission priority for the media data, and    -   performing retransmission of the media data received to the        client device, based on said transmission priority determined.

The method may further comprise storing said media data received, basedon said retransmission priority determined.

The method may further comprise the following steps:

-   -   receiving, from a client device implementing a method according        to the second aspect, an ordering update request,    -   updating said retransmission priority according to said ordering        update request, if said request is related to a media data to be        retransmitted, and    -   performing retransmission of the media data according to the        updated retransmission priority.

The method may further comprise the following steps:

-   -   receiving from a first client device, a request to a first        server device, for media data, wherein said media data is stored        by the proxy server for retransmission to a second client device        from a second server device,    -   determining priority values respectively associated with said        media data by said first and second server devices,    -   updating said priority values according to respective streaming        constraints for the first and second client devices, and    -   retransmitting said media data to said first and second client        devices according to said updated priority values, wherein said        first and second server devices implement a method according to        the first aspect and said first and second client devices        implement a method according to the second aspect.

The method may further comprise sending to the first and second serverdevices update notifications relating to the updated priority values.

According to another aspect of the invention there is provided a methodof streaming data between a server device and a client devicecomprising:

-   -   performing a method according to the first aspect by a server        device, and    -   performing a method according to the second aspect by a client        device.

According to yet another aspect of the invention there are providedcomputer programs and computer program products comprising instructionsfor implementing methods as defined above, when loaded and executed oncomputer means of a programmable apparatus.

According to yet another aspect of the invention, there is provided aserver device configured for implementing methods according to the firstaspect.

According to yet another aspect of the invention, there is provided aclient device configured for implementing methods according to thesecond aspect.

Solutions for adaptive streaming of media data from a server to a clientdevice have been proposed, in order to adapt in particular the type andquantity of data that are sent to the client device to the features ofthe concerned client device and to the characteristics of the networksproviding the connection between the server and the client device.

In this context, some solutions, such as the DASH (Dynamic AdaptiveStreaming over HTTP) standard, propose to store a plurality of versionsof the resource (or content) to be distributed and to send to a clientdevice requesting the resource a description file including adescription of the various versions representing the resource andrespective pointers (e.g. URLs) to these versions.

Based on the description file, the client device can then select aversion of the resource that best matches its needs and request thisversion using the corresponding pointer.

This solution is advantageous in that the description file is light asit contains no media data (but only pointers to media data). It avoidsthe exchange of media data that would be unsuitable for the clientdevice by letting the client select relevant versions for its usage.Moreover it fits in the current Web architecture based on HTTP and canexploit caching mechanisms already deployed.

In return, this solution however needs several exchanges (or roundtrips)between the client device and the server before media data is receivedat the client device and may then be decoded and displayed, whichresults in a start-up delay

In embodiments, the invention provides a method for providing media datarepresenting a media item (e.g. a video) from a server storing datarepresenting the media item, at least a temporal segment of which isrepresented by a plurality of versions, the method comprising thefollowing steps implemented by the server:

-   -   receiving a request from a client device for a description file        including a description of the versions representing the        temporal segment and respective pointers to the versions        representing the temporal segment;    -   selecting data among sets of data pointed to in the description        file;    -   sending the description file to the client device;    -   pushing the selected data to the client device.

By pushing data selected in an appropriate manner (i.e. sending datathat is not solicited by the client device, but has been selected by theserver as further explained below), one or several roundtrip(s) can beavoided and the decoding and display of the media data can thus startfaster.

The media item may for instance be a video, or an audio item, such as anaudio track, for instance.

It may be noted that the sets of data mentioned above include theversions representing the temporal segments, but may as well includeother data such as initialization data, as explained below.

As just noted, the selected data may include initialization data for adecoder of the client device. The decoder can thus be initializedwithout the client device having to specifically request for theinitialization data, and thus faster.

As noted above, the selected data may also include at least part of oneof said versions representing the temporal segment.

The step of selecting data may include estimating a quantity of data(e.g. video data) to be pushed, which may then be used when decidingwhich data are to be selected. The quantity may be estimated based on abuffer time defined in the description file and/or based on a bandwidthestimation determined by the server.

The step of selecting data may be performed based on at least onepreference included in the request and/or based on usage data derivedfrom prior exchanges between the server and the client device and/orbased on an analysis of the description file by the server and/or basedon a table stored in the server and associated with the descriptionfile.

According to a possible embodiment, it may be provided a step of sendinga push promise related to and prior to the step of pushing the selecteddata. The client device may thus be informed of the data to be pushed,before actually receiving these data.

The step of sending the push promise may be performed prior to the stepof sending the description file, which makes it possible to inform theclient device at an early stage.

The push promise includes for instance an identification of the selecteddata.

According to a proposed embodiment, the server determines a confidencelevel associated to the selected data and the push promise includes thedetermined confidence level.

According to a possible implantation explained in the detail descriptiongiven below, the server may store a hierarchical representation ofblocks of data forming the selected data. In such a case, the followingsteps may be provided:

-   -   receiving from the client device an instruction for not pushing        a block of data;    -   cancelling the pushing of said block of data and of blocks of        data connected to said block of data in the hierarchical        representation.

The proposed method may include a step of determining a level ofconfidence associated with the selected data; then:

-   -   if the determined level of confidence is below a predetermined        threshold, pushing the selected data includes pushing only        initialization data for a decoder of the client device;    -   if the determined level of confidence is above the predetermined        threshold, pushing the selected data includes pushing        initialization data for a decoder of the client device and at        least part of one of said versions representing the temporal        segment.

Embodiments of the invention also provide a method for receiving mediadata representing a media item (e.g. a video) from a server storing datarepresenting the media item, at least a temporal segment of which isrepresented by a plurality of versions, the method comprising thefollowing steps implemented by a client device:

-   -   sending a request to the server for a description file including        a description of the versions representing the temporal segment        and respective pointers to the versions representing the        temporal segment;    -   receiving the description file from the server, the description        file containing pointers to sets of data;    -   receiving unsolicited data from the server, wherein said        unsolicited data belong to said sets of data.

As noted above, the unsolicited data may include initialization data fora decoder of the client device (in which case a step of initializing thedecoder with said unsolicited data may be provided) and/or at least partof one of said versions representing the temporal segment (in which casea step of decoding at least part of the unsolicited data may beprovided).

The request may include at least one preference defining decoding at theclient device, which may help the server in determining the media datato be pushed.

The request may also include an indicator that the client device acceptspushed data, based on which the server may effectively decide to pushdata.

As explained above, it may be provided a step of receiving a pushpromise related to and prior to the step of receiving the unsoliciteddata. This step of receiving the push promise may occur prior to thestep of receiving the description file.

The push promise may include an identification of the unsolicited dataand/or a level of confidence associated with the unsolicited data.

The following steps may be provided at the client device:

-   -   determining acceptance or refusal of a push promise based on        data included in the push promise;    -   sending an instruction for not pushing said unsolicited data in        case of refusal.

The following steps may also be used:

-   -   determining acceptance or refusal of a push promise based on a        level of confidence associated with the unsolicited data and        included in the push promise;    -   sending an instruction for not pushing said unsolicited data in        case of refusal.

A step of buffering said unsolicited data upon receipt, before decodingthese data, may be used.

As pushed data are meant to correspond only to initialization dataand/or initial media data, the following steps may be implemented:

-   -   determining data (e.g. video data) to be requested (i.e. not        planned to be pushed) based on the description file and on data        included in the push promise;    -   sending a request for the determined data to the server.

Embodiments of the invention also propose a method for streaming mediadata representing a media item (e.g. a video) from a server storing datarepresenting the media item to a client device, at least a temporalsegment of the media item being represented by a plurality of versions,the method comprising the following steps:

-   -   the client device sending a request to the server for a        description file including a description of the versions        representing the temporal segment and respective pointers to the        versions representing the temporal segment;    -   the server receiving the request from the client device;    -   the server selecting data among sets of data pointed to in the        description file;    -   the server sending the description file to the client device;    -   the server pushing the selected data to the client device;    -   the client device receiving the description file from the        server;    -   the client device receiving the selected data from the server.

Embodiments of the invention also provide a device for providing mediadata representing a media item (e.g. a video) from a server, the serverstoring data representing the media item, at least a temporal segment ofwhich is represented by a plurality of versions, and comprising:

-   -   a receiver configured to receive a request from a client device        for a description file including a description of the versions        representing the temporal segment and respective pointers to the        versions representing the temporal segment;    -   a selection module configured to select data among sets of data        pointed to in the description file;    -   a module configured to send the description file to the client        device;    -   a module configured to push the selected data to the client        device.

Embodiments of the invention also provide a device for receiving mediadata representing a media item (e.g. a video) from a server storing datarepresenting the media item, at least a temporal segment of which isrepresented by a plurality of versions, the device comprising:

-   -   a module configured to send a request to the server for a        description file including a description of the versions        representing the temporal segment and respective pointers to the        versions representing the temporal segment;    -   a module configured to receive the description file from the        server, the description file containing pointers to sets of        data;    -   a module configured to receive unsolicited data from the server,        wherein said unsolicited data belong to said sets of data.

Lastly, embodiments of the invention provide a system comprising aserver and a client device for streaming media data representing mediaitem (e.g. a video) from the server storing data representing the mediaitem to the client device, at least a temporal segment of the media itembeing represented by a plurality of versions,

-   -   the client device comprising a module configured to send a        request to the server for a description file including a        description of the versions representing the temporal segment        and respective pointers to the versions representing the        temporal segment;    -   the server comprising a module configured to receive the request        from the client device, a selection module configured to select        data among sets of data pointed to in the description file, a        module configured to send the description file to the client        device and a module configured to push the selected data to the        client device;    -   the client device comprising a module configured to receive the        description file from the server and a module configured to        receive the selected data from the server.

Optional features proposed above for the method for providing media dataand the method for receiving media data also apply to the method forstreaming media data and to the various devices and system justmentioned.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent fromthe following description of non-limiting exemplary embodiments, withreference to the appended drawings:

FIG. 1a illustrates a graph of a set of resources owned by a server;

FIG. 1b illustrate an HTTP exchange without using a server PUSH feature;

FIG. 1c illustrates an HTTP exchange using a feature of pushing directlyconnected sub-resources by the server;

FIG. 1d illustrates a flowchart of an exemplary mode of operation on aclient side;

FIG. 1e illustrates a flowchart of an exemplary mode of operation of aserver implementing a push feature;

FIG. 2 illustrates exchanging multiple request and response prioritiesin parallel;

FIG. 3 is a block diagram illustrating Media streaming over HTTP;

FIG. 4a illustrates the generation of the media presentation and themanifest file;

FIG. 4b illustrates streaming protocol for MPEG/DASH;

FIG. 5 illustrates a Media Presentation Description (MPD) file;

FIG. 5a illustrates a flowchart of various steps associated with astreaming session by a DASH client;

FIG. 5b illustrates a tree representation of an exemplary manifest file;

FIG. 6 illustrates an example of a DASH manifest file;

FIGS. 7a and 7b illustrate media segment reordering according toembodiments;

FIG. 8 is a flowchart of exemplary steps performed by servers accordingto embodiments;

FIG. 9 is a flowchart of exemplary steps performed by clients accordingto embodiments;

FIG. 10 is a flowchart of exemplary steps performed by proxies accordingto embodiments;

FIG. 11 illustrates bandwidth measurement according to embodiments;

FIG. 12 illustrates video playing initialization according toembodiments;

FIG. 13 is a schematic illustration of devices according to embodiments;

FIG. 14a illustrates, using a flowchart, general steps of the inventionat the client side;

FIG. 14b illustrates, using a flowchart, general steps of the inventionat the server side;

FIG. 15a illustrates, using a flowchart, steps of determining the sharedpush policy at the client side based on the explicit approach;

FIG. 15b illustrates, using a flowchart, steps of determining the pushpolicy at the server side when the explicit approach is used;

FIG. 16 shows an MPD document in which a PushPolicy node is used tospecify a push policy applied by the server;

FIG. 17 illustrates, using a flowchart, steps for identifying andmarking some segments as ready to be pushed according to the shared pushpolicy “Push olicy”;

FIG. 18a illustrates an example of communication between a server andclient with a push policy transmitted in a HTTP “push-policy” header;

FIG. 18b illustrates the same example with a client's request to changethe push policy;

FIG. 19 illustrates an MPD document with a push policy which indicatesthat the three segments following a requested segment will be pushed;

FIG. 20 illustrates, using a flowchart, steps of the process at theserver side according to embodiments merging announcement messages;

FIG. 21 illustrates, using a flowchart, steps of the process at theserver side when using HTTP headers to declare the push policy;

FIG. 22 illustrates, using a flowchart, steps of the process at theclient side when using HTTP request to declare and share the pushpolicy;

FIG. 23 shows an MPD document in which a SupplementalProperty element isused to specify a push policy applied by the server at a hierarchicallevel of the document;

FIG. 24 shows an MPD document used as example for an XPath based pushpolicy;

FIG. 25 illustrates the reordering of elements in a priority tree, forexample in a webpage, before applying a push policy;

FIG. 26 shows exemplary methods respectively implemented by a server andby a client device to obtain a DASH fast start in accordance withembodiments of the invention;

FIG. 27 describes an exemplary method implemented by a server for a DASHfast start; and

FIG. 28 describes a possible method implemented by a client device for aDASH fast start.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In what follows, embodiments of the invention are described in thecontext of DASH-based networks implementing the HTTP 2.0 protocol. Thedata streamed is, for example, video data. Embodiments of the inventionare not limited to DASH networks.

A server device of a communication network that streams data to a clientdevice implements a push feature according to which it can transmit dataelements to the client without explicit requests from the client for thedata elements transmitted.

The server and the client may share push policies that drive the serverto determine the push promises and to actually transmit thecorresponding data. Thanks to this sharing, the client may anticipatethe push of some useless data, to cancel such push. This results inreducing the server's processing, as well as the network use sincePUSH_PROMISE frames may be cancelled before being sent.

In specific embodiments, the server can indicate in its push promises,by which it announces transmission of the not explicitly requested dataelements, ordering information concerning the order in which the serverintend to transmit the data elements. The order of the data elements maybe defined using priority values, for example the priority valuesaccording to HTTP/2.

Upon receipt of the push promises, the client device can determine inadvance the order of transmission intended by the server, therebyenabling the client to react to the proposed order in case it does notmatch its own desired order. For example, the client device can updatethe priority values and send the updated priority values to the server.The server can thus change the transmission ordering based on the newpriority values in order to better match the client's needs. The servercan use the updated priorities into account for future datatransmissions.

According to embodiments, the client may request a full reordering or apartial reordering of the transmission of the data elements to theserver.

Full reordering is described with reference to FIG. 7a . A clientrequests, during a step 700, a Media Presentation Description (MPDhereinafter) to a server. The server retrieves the MPD to send back tothe client and identifies corresponding data elements to push during astep 701. In the example of FIG. 7a , the server identifies “Data 1.1”,“Data 1.2” and “Data 1.3” as data elements to push. These elements arefor example data segments. Element “Data X.1” represents the base layerfor data X, element “Data X.2” represents the enhancement layer for dataX and “Data X.3” represents the additional enhancement layer for data X.The server defines a specific order of transmission for the dataelements. The server associates respective priority values with thePUSH_PROMISE frames to be sent to the client for announcing the upcomingpush data elements. The server then sends the PUSH_PROMISE frames“P1.1”, “P1.2” and “P1.3” with the associated priorities and the MPDduring a step 702. Next, shortly after sending the MPD and the pushpromise, during a step 703, the server sends to the client a data framecorresponding to the “Data 1.1” element and a PUSH_PROMISE messages“P2.1”, “P2.2” and “P2.3” respectively corresponding to the elements“Data 2.1”, “Data 2.2” and “Data 2.3”, which are segments following“Data 1.1”, “Data 1.2” and “Data 1.3” in the transmission order defined.In parallel to the receipt of the data frame and the push promise ofstep 703, the client decides, after receipt of the MPD and the “P1.1”,“P1.2” and “P1.3” PUSH_PROMISE frames, that the enhancement layer “Data1.2” is of lower priority compared to the additional enhancement layer“Data 1.3”. Thus, the client sends a priority update frame to lower“Data 1.2” priority during a step 704. Upon receipt of the priorityupdate request, the server changes the schedule of the transmissionduring a step 705. Hence, transmission of “Data 1.2” is postponed after“Data 1.3” is transmitted. In addition, the server uses the MPD to linkthe segments associated with “Data 1.2”. It identifies “Data 2.2” andlowers its priority as well.

Partial reordering is described with reference to FIG. 7b . Steps 710 to714 of FIG. 7b are substantially the same as steps 700 to 704 of FIG. 7a. After receipt of the priority update frame, the server behaviourdiffers as compared to step 705 previously described. During step 715,the server already started transmission of “Data 1.2” and proceedsfurther with the transmission. For that segment, there is no change inthe priority. The server nevertheless updates the priority of theconnected segments, namely “Data 2.2” in the present example. In orderto announce the fact that the priority change has been taken intoaccount, the server may send a priority update massage for “Data 2.2”.The client can thus be informed of the change.

Embodiments of the invention may be implemented in use cases whereinservers can push high quality video parts well enough in advance so thatthe whole part of the video can be played as high quality. For instance,the video can be split into a part 1, played as low quality, a part 2,played as high quality and a part 3 played as low quality. The bandwidthbetween the client and server allows real-time streaming of low qualitybut not high quality. In that case, the server may interleave part 1with the enhancement of part 2. Once part 1 has been played, theenhanced part 2 is also available and the server sends the base layer ofpart 2 to be played as high quality jointly with the enhancement of thesame part 2. Thus, the server makes sure that the whole part 2 is playedas high quality. Part 3 is sent thereafter. Quality flickering, whichdisturbs the user experience, can be alleviated and quality switchingonly occurs at a limited number of moments. The server is in the bestposition to know when to switch to a different quality level since itknows the video content.

FIG. 8 is a flowchart of steps performed by a server implementing apush-based DASH media streaming according to embodiments. Steps 800 to812 describe the general principles. Steps 820 to 827 more specificallydeal with the management of the priority feedback from the client.

During a step 800, the server receives a request R from the client. Thisrequest identifies a specific media, typically by referring to an MPDfile. Next, the server performs an iterative process comprising steps801 to 810. The process comprises sending data according to a definedorder. The order of transmission is updated according to the client'sfeedback. Once the data are sent, they are received and played by theclient. Next the server identifies new data to send and the processcontinues so on.

The first iteration starts with step 801, during which the data to besent are identified. In case of the first performance of the iterativeprocess, a fast start approach may be used in order to enable the clientto start video playing as quickly as possible. In addition, the servermay also identify subdivision of the media into chapters. In case theserver knows that the client generally navigates using chapters, theserver may actually select not only the segments that correspond to thebeginning of the media but also the segments corresponding to the startof the first chapters in the media. After the first performance of theiteration, the server may also detect that the connection may supportthe transmission of a higher quality representation of the media. Thus,the server may identify when the resolution or quality switch should bedone.

Once the server identified a list of segments to push, the serverdefines a transmission order for these segments. The transmission orderis used for computing initial priority values for each pushed segmentduring a step 802. The ordering may be based on several parameters.

A first parameter may be the relationships between the differentsegments: for example some segments must be available for correctlydecoding other segments. The segments that must be available are thusassigned higher priorities than said other segments.

A second parameter may be the popularity of video segments, which may begathered from past statistics. As an example, with YouTube URLs specifictimes in a video may be addressed. When clicking on the links associatedwith these URLS, only the video needed to start the video playing at thespecified time is retrieved. In addition, if a video is being chaptered,the beginning of each chapter is generally more often retrieved fromusers than segments in between chapter starts. Segments of the chapterbeginning are thus assigned higher priorities than the in-betweenchapter segments.

A third parameter may be the timeline: the priority of a video segmentthat is closer to being played is higher than the priority of a videosegment that is to be played later.

A fourth parameter may be the estimated time spent to actually transmitthe segment. When the video segment is large, it takes a long time to betransmitted and therefore, transmission should start as soon aspossible, i.e. with a high priority.

In case two segments have identical priorities, the corresponding dataframes can be interleaved during transmission.

In case regions of interests are identified in the media content, if thebandwidth is not large enough for a high quality representation but islarge enough for a low quality representation, the server may select anenhancement layer only for the region of interest.

Once the priorities are computed, the server sends PUSH_PROMISE framescontaining the priority values during step 803. Identification of allsegments is not needed for starting transmission of the PUSH_PROMISEframes. In case an MPD is to be sent for the segments to be pushed (step804), the MPD is sent (step 805). The segment transmission starts inparallel during step 806.

Once the PUSH_PROMISE frames are received by the client, the server mayreceive priority update changes and then change its transmissionschedule accordingly (steps 807 to 808 and steps 820 to 828). Whilesending segments, the server awaits receipt of priority change messages.In case a priority change message is received (step 807), the serverreorders the segments accordingly and continue the segment transmission(step 808). Once all segments are sent (step 809-1), the server restartsan iteration process in order to continue streaming the media until theend of the media. When the end of a media is reached (step 809-2), theserver checks whether or not it should automatically start streaminganother media (step 810). In case another media should be streamed(Yes), the server identifies the new media to stream (step 811) andrestarts the process from step 801. In case no new data should bestreamed, the process is stopped (step 812).

The management of the priority feedback from the client, i.e. of step808, starts with the receipt of a priority update change message duringstep 820. The following steps may also be performed in case the clientcancels a segment push: this case may be seen in practice as equivalentto assigning the lowest priority to that segment.

Upon receipt of the priority update change message, the serveridentifies the related segment during step 821. The server then proceedswith the reordering of the segment transmission (steps 822, 823). If thesegment is already transmitted, the process ends. If the segment isbeing transmitted, depending on the server implementation, it may refuseto change the transmission (for example because it is too complex) or itmay actually reschedule the remaining data to be sent.

The rescheduling of the data may be performed as follows. The serverstores a list of video segments to push (and/or video segments that arebeing pushed). This list is ordered according to the priorities set bythe server. The server then sets the new priority value for the segment.The list is then reordered and the corresponding video segmenttransmission is made earlier or later accordingly.

Once the video segment is reordered, the server may actually decide toapply this priority change to other related video segments. If a clientraised the priority of a video segment which is part of an enhancementlayer, the server may raise the priority of all the segments of thisenhancement layer. Conversely, if the client lowers the priority of abase video segment layer, the priority of all segments temporallyrelated to this segment may be lowered. This process is described insteps 824 to 827. Based on the MPD and the rescheduled video segment,the server identifies a list of related segments (step 824). Therelationship may be temporal, spatial, quality-based etc. The MPD may beenhanced in order to better show the potential relationships. Inparticular, when the priority of an initialization segment (which isnecessary to play more than one video segment) is lowered or raised, allrelated segments may be rescheduled. This can be the case as well forbase layer segments and enhancement segments. For each identifiedrelated segment, the server tests whether or not the transmission of therelated segment should be changed (step 825). In case it should bechanged, the server computes the new priority value for each segment(step 826) and reschedules the segment transmission accordingly (step827). The new priority value may be computed by adding to the old valuethe difference between the new priority value received during step 820and the initial priority value of the segment identified during step821. The process stops when each related segment has been tested (step828).

The server may also receive control flow messages, such as WINDOW_SIZEframes. These messages may enable the server to identify what the clientis currently playing. When some additional buffer space is available onthe client's end, it may be inferred that some data have been removedfrom the buffer, typically the oldest data. If the server keeps ahistory of the data sent, the server is able to identify which data havebeen removed. Thus, provided the server knows the client's cacheordering, the server can have knowledge of which video segments theclient is currently playing. This ordering may be based on the MPD thatmakes it possible to order the cached data according to the timeline. Aserver may then detect client time skipping for instance. The server mayreact by quickly sending the start of the next chapter in advance sothat the client can continue skipping video chapters.

It should be noted that the sending of a PUSH_PROMISE frame withpriorities may be done in various ways. A PUSH_PROMISE frame must relateto an opened stream which is initiated by the client. According toembodiments, the initial stream made by the client during step 800 maybe always left open. According to other embodiments, a PUSH_PROMISEframe is sent within a stream opened by the server. In this case, theclient considers the PUSH_PROMISE frame as it is sent by the parentclient-initiated stream. Thus, it can compute the right headers of thevirtual request corresponding to the particular PUSH_PROMISE frame.

According to other embodiments, a priority message is sent jointly witha PUSH_PROMISE. A first possibility is to send it as a header within thePUSH_PROMISE frame. Another possibility is to send a PRIORITY frame withthe stream ID reserved by the corresponding PUSH_PROMISE frame. A thirdpossibility is to send the PUSH_PROMISE frame, then the correspondingHEADERS frame (to open the stream) and then the PRIORITY frame on thisnewly opened stream.

In order to further control the client's buffer, the server may send anew representation of a segment cached by the client. Within the headerssent as part of this new representation, HTTP cache directives may beused to request the client to actually remove the segment, for instanceby marking it as not cacheable. This may make it possible to recoverbuffer space on the client's end. HTTP/2 control flow may be used. Theserver can then push additional data.

A server may send priority values for each video segment. The server mayalso send priority values for specific segments. In case the server didnot send a priority value for a current PUSH_PROMISE frame, the clientcan compute a priority value from the last priority value sent from theserver. For instance, the client may increment the priority value eachtime a new PUSH_PROMISE frame with no priority value associated with isreceived. Hence, the PUSH_PROMISE frames can be grouped so that updatingthe priority of the specific segment will also update the priorities ofall segments of the group.

The process on the client's side is described with reference to FIG. 9.

The client should be able to play the content available at a given time.However, the client has to cope with potential buffer limitations andprocessing time. The client has to check whether or not the transmissionordering proposed by the server matches the memory space available inthe client's buffer and matches the content currently played by theclient.

During a first step 900, the client connects to the server and requestsan MPD file. The client then retrieves the MPD file during a step 901and waits (step 902) for the receipt of data. When data are received,the client checks (step 903) whether the data are a push promise. Incase a push promise has been received, this means that a new videosegment is being sent by the server. The client processes the pushpromise. In particular, the client may validate the priority valuesproposed by the server during step 904. In case the client wishes tochange the priority values (step 905) for the current segment or anotherpromised segment, the client compute a new priority value and sends itto the server (step 906).

In case the client receives video data (step 907), the client links thevideo segment to the MPD file (step 908) and stores the video data (step909). Linking the video data to the MPD file makes it possible for theclient to retrieve the video segment when it will be further used fordecoding the video (step 911). This may also provide efficient storageof the video data (step 909), for example if contiguous video segmentsare grouped.

The buffer storage constraints may further change the priority. Thus,the client may check again whether a priority value has to be changedand may communicate with the server if needed (steps 905, 906).

Once the client is ready to start or continue playing video (step 910),the client retrieves from its cache the next time slot video segments(step 911) and decodes and plays the video (step 912). As part of step911, the client may query its cache in order to know which videosegments are available. By default, the client may use all videosegments available, in particular all enhancement segments if any. Theclient may let the server select the content: generally speaking, allsegments should be used by the client. If some segments cannot be usedjointly (like audio English tracks and French tracks), the client shoulddismiss the unused segments in the first place. It should be noted thatnot all clients may get access to the cache state: web applications inparticular do not usually have access to the web browser cache. In sucha case, the server may directly send the list of pushed segments to theweb application client. For instance, this information may be exchangedfrom the server to the client using a web socket connection.

As the video is played and decoded, the corresponding video segments maybe removed from the buffer. Hence, the client updates its availablebuffer size using a WINDOW_SIZE frame. The client may keep videosegments that have been recently played in order to enable the user torewind the video during a limited period of time. The flow controlupdate mechanism may also be used when the user does a fast forward/timeskip. The client may remove old stored video content to make room fornew content and announces this change to the server using a WINDOW_SIZEframe. When the server receives the WINDOW_SIZE frame, the server may beable to compute which video segments were removed and then identify whatthe client is actually playing, as discussed above.

In what follows, step 904 is described in more details.

The client holds a list of all push promised video segments. This listis ordered according to the priority information found in the pushpromise frames. First, it is checked for potential frozen video issues.Based on an estimation of the available bandwidth and the ordered videosegment list, transmission beginning and end times of each segment canbe estimated. Based on these times, it may be tested whether each videosegment will be available at the time it should be used for videoplaying. If a promised video segment is expected to be delivered afterits corresponding video playing use, its priority should be increased.Thus, the video segment is moved up in the push promised video segmentlist order. In order to compute the exact priority value, it is searchedfor the position in the video segment list that makes it possible tohave the video segment delivered on time and that is the closest to thecurrent video segment position. The priority is then set to a valuebetween the priorities of the video segments in the list that are beforeand after the video segment new position.

Other factors may also be used by the client for changing the videosegment priorities. For instance, if the client is expecting to do somechapter-switching, the client may actually increase the priority of allvideo segments that start the chapters, in particular the correspondinginitialization segments.

According to embodiments, the client-side flow control comprisesdisabling the per-stream flow control and keeping only a per-connectionflow control. The per-connection window size defines the maximum amountof video that a client may actually store at any given time. The clientand the server may negotiate at initialization time and during theconnection in order to decrease or increase this window size. If theserver wants to push some HD content, the server may request the clientto increase the window size. If the connection bandwidth is low, theserver may need to anticipate well in advance the sending of HD contentfor a specific part of the video, in which case the buffer size shouldbe made larger.

The order of transmission may be an important issue when the buffer hasa single size. In particular, as the buffer is filled with data, thepriority ordering becomes more and more important. An importantconstraint is that the video never freezes. As long as the buffer islargely empty, the server may push various video segments, like segmentslargely in advance in order to provide an efficient fast forward orchapter skipping. Once the buffer is almost fully filled, the videosegments to push should be as close as possible to the video segmentsbeing played. This push behaviour may be done by the server if theserver has accurate information concerning the client buffer. It mayalso be implemented by the client using the priority update mechanism.

In case of automated video switching, the flowchart of FIG. 9 may beextended by detecting the push of a new MPD as part of the push promisecheck (step 903). When an MPD push is detected, the client may startreceiving segments of a new video as part of step 908. The client musttherefore identify the MPD related to the video data. Once the videoplaying is finished for a given MPD (step 902), the new MPD may be usedfor continuing video playing. The client may actually flush all videosegments linked to the previous MPD.

With reference to FIG. 10, the behaviour of a DASH-aware proxy isdescribed. When receiving a segment pushed from a server, a proxy is notmandated to push it to the end-client. In case of DASH streaming though,it can be considered good practice (or default behaviour) to do so.

The proxy may be able to adjust the server and client behaviours, bothin terms of priority processing as well as pushed data to be sent. Aproxy may in fact handle independently the priorities with the clientfrom the priorities with the server. In addition, the server may pushmore data than needed for a given client and the proxy may retrieve theadditional pushed data to fulfil requests from other clients.

A server may push a video segment for several reasons. For example, avideo segment may be pushed in case it is believed to be useful for theend-client. A video segment may also be pushed in case it is believedthat the video segment can be used several times and that it is worthpushing it to proxies.

In the first case, proxies generally send the video segment to theclient. Proxies may postpone its transmission in order to optimize theclient or proxy network state, for instance the client radio state. Anexemplary case may be the segment push for fast start video playing andbandwidth estimation, in which case data should be sent as fast aspossible to the client. In case the server is interested in pushing datato proxies, proxies may not automatically send the video segment to theclient, except if they have means to know that the video segment will beuseful to the client. In order to make possible the identification ofvideo segments that may not be sent to clients, a specific priorityvalue may be used. Using a priority value makes it possible to have theproxy always check the priority value for optimizing the processing ofthe various frames that arrive.

FIG. 10 comprises three flowcharts. One flowchart relates to the processof filtering pushed segments (steps 1000 to 1008). Another flowchartrelates to the process performed when a segment is requested by a clientwhile it is already promised to another client (steps 1010 to 1015).Another flowchart relates to the management of priority changes (steps1020 to 1026).

The process of filtering pushed segments starts with the receipt (step1000) of a pushed data event, typically when receiving a PUSH_PROMISEframe or a related DATA frame. The proxy checks whether the data are ofhigh priority or not (step 1001). Data may be considered as of highpriority if their priority value is much larger than priority values ofother segments being transmitted. Data may also be considered as of highpriority if its priority value has a special meaning, such as fast startor bandwidth estimation. If the data are of high priority, they are sentas quickly as possible to the client (step 1002). The proxy then decideswhether or not to store the data (steps 1003, 1004). This decision maybe made once when receiving the corresponding PUSH_PROMISE frame or thecorresponding HEADERS frame that opens the pushed data stream. Thisdecision may also be based on the proxy cache state, the envisioned useof the video, the popularity of the video source or other criteria. Theproxy stores the video segment if the segment is pushed while beingrequested by one or more clients at the same time. The video segmentsmay also be stored if segments are identified as fast start.

If the data are not of high priority, the proxy checks whether it is oflow priority (step 1005). Data of low priority may be data for whichtransmission to the client may be skipped but that are considered by theserver as interesting for network intermediaries like proxies. The proxyfirst decides whether or not to send the data to the client (step 1006).This decision may be made once when receiving the correspondingPUSH_PROMISE frame or the corresponding HEADERS frame that opens thepushed data stream. If it is decided so, the proxy sends thecorresponding frame to the client (step 1002). The process then stopsafter deciding whether or not to store the data.

The priority value negotiated between the server and proxy may bedifferent from the priority value negotiated between the client andproxy. Therefore, in case the data is of usual priority (i.e. not of lowpriority and not of high priority), the proxy checks whether the segmentpriority value is managed by the proxy. As illustrated in FIG. 10 (steps1020 to 1026), the proxy uses the client-to-proxy value for schedulingthe time when the data should be transmitted: the proxy holds a list ofall to-be-transmitted video-related frames. These frames are orderedaccording to the priority values before being sent following that order.

In the case the proxy is receiving a priority update frame (step 1010),the proxy identifies the related video segment (step 1011). If itspriority value is not being managed by the proxy (step 1012) the proxyforwards the priority update frame to the server (step 1013). Otherwise,the proxy stores this new priority value and reorders the video segmenttransmission (step 1014) accordingly. In case a potential conflictappears, in particular in case the video segment delivery from theserver is expected to be too late for the client needs, the proxy canthen forward the priority value to the server.

Steps 1020 to 1026 relate to the case of a proxy that receives a requestfrom a client to a video segment (step 1020) that is already promised bythe server to another client (step 1021). Depending on the prioritygiven to that request, the proxy computes the minimum proxy-to-serverpriority that would fulfil the client's request (step 1022). Thiscomputation is done by computing the proxy-to-server priority value thatwill ensure that the server-to-proxy delivery time is earlier than theproxy-to-client expected delivery time. The priority is changed if thecomputed priority is below the currently set priority (step 1023), inwhich case the proxy will send a priority update message to the server(step 1024) and the proxy will mark this video segment priority asmanaged by the proxy so that the proxy sends the video segment to itstwo clients at the best time for their needs. Similarly to this process,a proxy may receive several priority updates to the same segment fromseveral clients, in which case the proxy may actually send the lowestpriority value that satisfies all clients.

With reference to FIG. 11 there is described an embodiment according towhich a client receives a pushed data event whose priority valueindicates that the server wants to use it for measuring bandwidth.Measuring bandwidth may be done using TCP/IP packets through active orpassive measurements for computing round trip times. Based on round triptimes, the available bandwidth may be computed as found in documentSaubhasik et al. “Bandwidth Estimation and Rate Control in BitVampire”.This computation may potentially take into account effects of HTTP/2control flow. By making notification that some data frames are used forbandwidth estimation possible, the bandwidth available without HTTP/2control flow can be estimated.

The process starts with step 1100 during which a pushed data frame isreceived from the server. Next, it is checked whether the associatedpriority of the stream indicates that the server is measuring bandwidth(step 1101). In that case, the dedicated buffer is maximized (step1102). Alternatively the stream flow control can be disabled. If thereceiving node is a proxy (step 1103), it may forward the segment data.Otherwise, the client decides whether to store the segment (step 1104).The client stores the pushed segment (step 1105). In any case, theclient sends an acknowledgement to the server in the form of aWINDOWS_UPDATE (step 1106) for the per-connection window. Thisacknowledgment will then be used by the server for estimating theconnection bandwidth. In the case the client is a proxy, it forwards thepushed data (step 1108) as quickly as possible. When receiving anacknowledgment from the end-client, the proxy forwards it back to theserver as well (steps 1109, 1110).

In order to estimate the available bandwidth, the server may use theround trip time of the sent data frame that is computed as thedifference between the sending time of the data frame and the receptiontime of the acknowledgment message, the pairing between the two beingbased for instance on the data frame size which should be equal to thewindow size update. Round trip times can be computed from various dataframes of one or more video segment. In order to increase accuracy, thedata frames may have various sizes. Splitting a video segment intoseveral DATA frames of different sizes can be performed by the server.The server only needs to ensure that the network layer will not splitDATA frames into several TCP/IP packets (hence smaller DATA frames) ornot buffer content to be sent and merge several DATA frames into aTCP/IP packet. Based on those measurements, standard techniques can beused for computing the available bandwidth (an example can be found inthe above-mentioned document) that the server may use to actually decidewhich video representation to use.

With reference to FIG. 12, there is described the case of an initialvideo playing. The server pushes data using the fast start priority. Itis considered that the data probably have a low-bit rate and that theclient will receive those data and send acknowledgments to the server sothat the server can estimate the bandwidth and switch to the optimalrepresentation. The client-side process is described in steps 1200 to1207. The server-side process is described in steps 1210 to 1215.

The client process starts with a step 1200 of receipt of pushed data.The client then checks whether the priority has the fast start value(step 1201). In that case, the client typically maximizes the dedicatedbuffer (step 1202). This maximization is performed when receiving thePUSH_PROMISE of the pushed data. The data are then stored (step 1203)and the client sends an acknowledgement to the server using theWINDOW_UPDATE frame (step 1204). The client then checks whether enoughdata are available to start playing the video (step 1205). If they are,the video playing starts (step 1206). Otherwise the client waits formore data (step 1207), until enough data are available for startingplaying the data.

The server process starts with a step 1211 of sending segment dataframes with the fast start priority (step 1210). The server thenreceives acknowledgments (step 1211) that will allow computing theavailable bandwidth (step 1212). Once enough measurements are obtained,the server selects the optimal representation (step 1213) and startspushing optimal representation segments (step 1214). The server decideswhen to switch representation. This has at least two benefits. First theserver may know when the measurements are accurate enough and may switchfrom one resolution to another as soon as this is the case, while theclient will need to handle some delay. Second, the server may decide toswitch from one resolution to another at the time that is lessdisturbing for the user experience. Indeed, the server has the knowledgeof the video content. In particular, the MPD may be augmented withinformation on the times at which resolution switch can be bestenvisioned.

The present invention relates to an enhanced streaming method where, atthe server side, a request relating to first media data is received,from the client device; second media data to be sent to the clientdevice without having been requested are identified; and then datarelating to said first media data are transmitted to said client device,in response to said request, and at least one announcement messagerespectively identifying said second media data is prepared with a viewto transmit the announcement message or messages to the client device.

At the client side, a request relating to first media data istransmitted to the server device; and data relating to said first mediadata are received from said server device, in response to said request,

The enhanced streaming method reduces the mismatches between theserver's decisions to push some media data and the client's needs forsuch data. As will be apparent from below, the server and the clientshare a push policy so that they both determine the same media data tobe pushed from any media data requested by the client. A push policydefines how to determine the data to push, and may be seen as a rule fordetermining which resources linked to requested data are about to bepushed after the requested data are processed (after a GET request), andpossibly how they are pushed (e.g. in which order). Usually, the linkedresources are determined using one document, for instance a manifestfile, such as an MPD file (in the DASH context for multimedia data), oran HTML document.

As a consequence, based on the shared push policy, the client is able toanticipate the server's behaviour to avoid, and more precisely tocancel, the transmission of useless media data from the server. Use ofbandwidth in the communication network between the client and the serveris thus reduced. Furthermore, the number of HTTP requests andPUSH_PROMISE cancellation is reduced, which lowers the latency of theapplication in particular for low latency live video streaming.

According to the invention, the server may use a push policy shared withthe client device for the server device to drive the identification andthe transmission of the second non-requested media data to the clientdevice. In particular, it may use a push policy shared with the clientdevice and defining how to determine second media data, for the serverdevice to determine the second non-requested media data to be sent tothe client device. Correspondingly, the client may use a push policyshared with the server device and defining how to determine second mediadata, for the client device to determine second media data to be sent bythe server device without having been requested by the client device.

FIG. 14a illustrates, using a flowchart, general steps of the inventionat the client side, while FIG. 14b illustrates, using a flowchart,general steps of the invention at the server side.

In comparison to the process described with reference to FIGS. 1d and 1e, additional stages 1400 and 1402 make it possible for respectively theserver and the client to determine the push strategy that is shared withthe other, and thus to be used.

According to first embodiments, the shared push policy is an implicitpush policy, meaning that the client and server do not exchange(explicit) policy data to tell the other what is the push policy to beshared. An implementation of the implicit approach for the shared pushpolicy includes using the same algorithm, referred to as “second mediadata determining algorithm”, at both the server device and the clientdevice, the algorithm enabling the server device and the client deviceto determine the same second media data from the first media datarequested.

For instance, the algorithm is predetermined either during the set-up ofthe client and server or relatively to a specific standard. A typicalexample of algorithm may consist in pushing the N resources followingthe requested resource in a parsing order of the manifest file, where Nis a predetermined number, for instance 4.

Referring to the Figures, the steps 1400 and 1402 consist, in case ofimplicit push policy, in loading in memory the predetermined algorithmfor identifying the resources to be pushed (step 1403 at the serverside).

The client may efficiently use the so determined push policy forestimating the number of PUSH_PROMISE that are expected and forpreparing cancellation messages for unwanted push data, for instance instep 1401.

For instance, this would result for the server in receiving, from theclient device, a cancellation request requesting to cancel transmissionof a part of the second non-requested media data so that the serverdevice does not transmit the corresponding prepared announcementmessage. For its part, the client would thus send, to the server device,a cancellation request requesting to cancel transmission of a part ofthe second non-requested media data, in order to drive the server devicenot to transmit an announcement message identifying the part of thesecond non-requested media data. One may understand that suchcancellation can occur before the announcement message is transmittedfrom the server device or received by the client device. This approachmay be useful for example, when the client decides to switch to anotherversion of a medium. In such situation, it can decide to cancel thesegments pushed for the previous version.

It may also be noted that, thanks to the knowledge of the resources tobe pushed using the algorithm, the client can make a second request tothe server in parallel, in order to retrieve subsequent resourceswithout having to wait for the corresponding PUSH_PROMISE from theserver. In case of DASH, this possibility for the client makes itpossible to reduce the latency of the client while ensuring that thesecond request will not interfere with a PUSH_PROMISE that will bereceived later.

The client may also request other resources that it needs, if itdetermines from the results of the algorithm that these other neededresources are not about to be pushed.

According to second embodiments, the shared push policy is defined inexchanges between the client and server, either explicitly by definingthe whole rule (i.e. algorithm or parameters of the algorithm), or usingreferences to push policies predefined at both sides. This requires forthe server to first determine a push policy information describing thepush policy of the server. Then the push policy information istransmitted to the client to share the push policy with the client.Correspondingly, the client thus receives, from the server device, pushpolicy information describing the shared push policy.

One advantage of the explicit approach relies on the fact that adifferent push policy could be used by the server for each client or foreach multimedia presentation (e.g. each MPD), in order to better meettheir processing characteristics. FIG. 15a illustrates, using aflowchart, step 1400 of determining the shared push policy at the clientside based on the explicit approach, while FIG. 15b illustrates, using aflowchart, step 1402 of determining the push policy at the server sidewhen the explicit approach is used.

As shown in FIG. 15b , the server generates at step 1504 a message todeclare the push policy and then sends it to the client at step 1505, inorder to share it. The information describing the push policy in thedeclaration message is referred to as push policy information.

FIGS. 16 to 18 described below give exemplary details on how the pushpolicy is declared and transmitted to the client.

The resources to be pushed using the push policy as determined at step1402 are then identified at step 1403 with the selection algorithm (orsecond media data determining algorithm) defined in the push policydeclaration message generated in step 1504.

On the client side as shown in FIG. 15a , the client is able topre-identify the resources to be pushed for a given resource request byapplying the same selection algorithm. This makes it possible for theclient to predetermine the data that will be pushed by the server andthus ensure an efficient management of push data and a reduction in thenumber of GET requests if appropriate.

To apply the same selection algorithm, the client receives the pushpolicy information describing the push policy applied by the server.

Various push policy declaration methods may be used.

In one embodiment, the push policy declaration is shared thanks to aJavaScript program that takes, as input parameters, a request R and aDOM tree corresponding to the document containing the resources to bepushed (typically the manifest file for DASH) and that outputs anordered list of resources to be pushed. In this embodiment, the pushpolicy information includes a JavaScript program embedded in a web pagetransmitted from the server device to the client device.

In other embodiments, the push policy is described within the manifestfile. That is the push policy information describing the shared pushpolicy is inserted in a description file that is transmitted from theserver device to the client device using the shared push policy. Thedescription file contains description information that concerns mediadata including the first media data, and is used by both sides todetermine the second non-requested media data to be pushed.

In DASH, the description file is for instance the MPD file. Thedescription below is mainly based on DASH and MPD files. However, thesame approach applies to other manifest-based streaming methods likeSmooth Streaming or HTTP Live Streaming.

According to specific embodiments, the push policy information includesa first push attribute defining an amount of second non-requested mediadata to be identified in a description file. This makes it possible tospecify the number of segments to be pushed after one request R isreceived from the client.

This is illustrated by FIG. 16 which shows an MPD document in which aPushPolicy node 1600 is used to specify the push policy applied by theserver.

In this example, the PushPolicy node 1600 includes a push attribute,namely “SegmentIdx”, to declare the number of segments to be pushedafter a GET request is received. For instance, if the client requestssegment 1601 in its GET request, it will receive, as a response, aPUSH_PROMISE frame for the next two segments in parsing order of the MPDdocument. In this example, the first push attribute identifies thesecond non-requested media data relatively to the first media datarequested within the description file. More generally, a predeterminednumber of K segments to be pushed is used to define the push policyvalue. Consequently, for each segment requested by the client, theserver will push the K next segments.

Whereas the example 1600 of FIG. 16 shows a single push attribute, theremay be several push attributes. Each push attribute may represent aconstraint on nodes of the DOM (Document Object Model) tree representingthe manifest for selecting the segments to be pushed. Referring to theprevious example of FIG. 4b , the push policy node 1600 can refer tomedia data described in the description file (MPD file) using media dataattributes (the MPD elements and/or attributes) including a periodattribute “PeriodIdx” that refers to a Period element to which the mediadata belong, an adaptation attribute “AdaptationSetIdx” that refers toan AdaptationSet element of the media data, a representation attribute“RepresentationIdx” that refers to a Representation element, i.e. anencoding version (specific codec, resolution or bitrate . . . ) of themedia data and a segment attribute “SegmentIdx” that refers to a segmentin a given Representation.

Based on these existing media data attributes, the push policyinformation may include at least a second push attribute defining aconstraint on the media data attribute or attributes, for identifyingthe second non-requested media data.

For instance, a push attribute may be related to the PeriodIdx attributeto specify a constraint on the period for selecting the segment to push;another one may be related to the AdaptationSetIdx attribute to specifya constraint on the adaptation; another one may be related to theRepresentationIdx attribute to specify a constraint on therepresentation; in addition to the above first push attribute related tothe SegmentIdx attribute.

When a push attribute is not present or void, the related media dataattribute must be considered as unconstrained.

The value of the push attributes may use the following syntax:

push attribute=[operator] operand

where “operator” is optional and takes the value ‘+’ or ‘−’ to definethe segments to be pushed relatively (“+” means after and “−” meansprior to) to the requested segment, and where “operand” is either aninteger value superior or equal to 0 or ‘*’ as wildcard parameter.

FIG. 17 illustrates, using a flowchart, steps for identifying andmarking some segments as ready to be pushed according to the shared pushpolicy “PushPolicy”. This flowchart illustrates step 1403.

First, the server identifies at step 1700 the segment requested in themanifest file. The request includes an identified “reqSegIdx” of thissegment.

For each node type in the manifest file MPD, an index value isattributed to each node. The value is incremented for each Node in theorder of appearance in the manifest file.

Next, the indexes of the Period, AdaptationSet, Representation andSegmentURL that correspond to the requested segment (i.e. the segmentspecified in the GET request) are retrieved by parsing the whole MPDuntil the requested segment is reached.

The operator and operand values of the push attributes defined in thepush policy are used to identify in which nodes the segments to bepushed are defined (except for the SegmentIdx attribute that defines theamount of segments to be pushed, when associated with “+” or “−”operator).

When no operator is specified, the operand value identifies the index ofthe Node within which the data to be pushed have to be retrieved. Forexample, where the first push attribute “SegmentIdx” has no operator, itis an identifier, within the description file, of a specific segment tobe pushed. In one alternative, when the operator is not specified theoperand value may identify range values, for example “SegmentIdx=2-5”would return segments with index equal to 2, 3, 4 and 5.

Otherwise (an operator is specified), the operand value represents anoffset value (named “idxOffset”) to apply to the index of the requestedsegment (“reqSegIdx” obtained in step 1700). In such case, the segmentsto be pushed should be in the Nodes with indexes comprised in the[reqSegIdx, reqSegIdx+idxOffset] range if the operator is “+” and in[regSegIdx−idxOffset, regSegIdx] if the operator is “−”. The use of anoperator makes it possible to define a media data attribute orattributes of the second non-requested media data relatively to thecorresponding media data attribute or attributes of the first media datawithin the description file.

For instance, let's consider the following push policies:

-   -   1. <PushPolicy RepresentationIdx=“−1” SegmentIdx=“2”/>    -   2. <PushPolicy PeriodIdx=“+1” SegmentIdx=“+2”/>    -   3. <PushPolicy PeriodIdx=“+0” SegmentIdx=“+2”/>

PushPolicy #1 specifies that the server will push the segment of index 2in the representation node preceding the representation node of therequested segment.

With PushPolicy #2, the server will push the two segments following therequested segment, either in current period or in the following. Forexample, when requesting segment 2401 on FIG. 24, segments 2405 and 2402would be pushed.

PushPolicy #3 is very similar to the PushPolicy#2, the main differenceis when the requested segment is the penultimate of the Period. Forexample, when requesting 2401, only the last segment 2405 in the currentPeriod (instead of two segments) would be pushed. With PushPolicy #3,PeriodIdx restricts the segment search to the requested segment's Periodnode and thus only the last segment of the Period is pushed (because thesegment requested is the penultimate segment in the Period). On thecontrary, with PushPolicy #2 the segments can be retrieved from the nextperiod.

In an alternative or as an optional value, the value of the operand maybe also ‘*’ (wildcard meaning) which means that any segment should bepushed. When it is associated with operator ‘+’ (respectively “−”), itmeans that all subsequent (resp. preceding) segments to the requestedone should be pushed.

This alternative allows the client to send only a single HTTP request toretrieve all segments of one Period for instance with the followingPushPolicy: <PushPolicy PeriodIdx=“+0” SegmentIdx=“+*”>.

In these examples, the use of the SegmentIdx attribute to identify thesecond media data (to be pushed) relatively to the requested first mediadata requires that the second media data are adjacent to the first mediadata. In an embodiment, the SegmentIdx attribute may include an offset(in addition to the operand) to apply to the index of the requestedsegment. This shifts the index of a reference segment from which aspecified amount of segments has to be pushed. As an example, the syntaxof the SegmentIdx attribute may be:

push attribute:[operator]operand[,offset]

where “offset” is a positive or negative integer different from 0 toapply to the requested segment index. In such case the search range is[reqSegIdx+offset, reqSegIdx+idxOffset+offset] when the operator is ‘+’and [reqSegIdx−idxOffset+offset, reqSegIdx+offset] when the operator is‘−’.

The syntax of the push policy can also contain conditions like (nonrestrictive) a maximum size of data or a time in the presentation beingpushed, respectively. For instance:

<PushPolicy SegmentIdx=‘+*[size<500000]’> defines a push policy to pushno more than 500 kilo bytes of segments data.

<PushPolicy SegmentIdx=‘+*[time<0:01:30]’> defines a push policy to pushno more than 1 minute and 30 seconds of next segments data.

While the above examples show how to declare the push policy thatdetermines which segments have to be pushed, there may be a need to alsospecify in which preferred order the segments will be pushed. Thisinformation should also be shared between the client and the server.

As an example, the declaration of an order of transmission of the pushedsegments as described above with reference to FIGS. 7 to 12 could apply.

In one alternative embodiment for an order of transmission of pushedsegments, the description information in the description file includespriority attributes associated with the media data, one priorityattribute (for example “priorityIdx”) for each media data, and an orderof transmission of the second media data is based on the associatedpriority attributes. Thanks to the transmission of the description file,the client is also aware of the values taken by these priorityattributes, and is thus able to determine the intended order oftransmission.

As shown in the example of FIG. 16, each segment (for instanceidentified by one SegmentURL Node) described in the manifest fileincludes a priorityIdx attribute (1604) which specifies the push orderof the segment. In the example of FIG. 16, segment 1603 is pushed beforesegment 1602. These priorities are computed during the media segmentspreparation at the server side. Different priority values can be used: arelative priority value in a given Representation (as on FIG. 16) or anabsolute priority value either as a 32 bit number with 4 mostsignificant bits for the Period priority, 4 next MSBs for theAdaptationSet priority value, next 8 bits for the Representationpriority value and least 16 significant bits for the segment priority.An alternative way of signalling the absolute priority value is to use acomma separated list of priority values, one for each of the above citedlevels, for example: priorityIdx=‘1, 1, 2, 1’ to define successively thePeriod priority, the AdaptationSet priority, the Representation priorityand then the segment priority. First embodiment with 32 bit value wouldgive (in binary):

priorityIdx=‘00010001000000100000000000000001’.

A main advantage of using priorityIdx values is to make it possible todefine a priority order between segments from different Representation(typically an associate representation such as an alternate view of thevideo). It is useful when the push policy consists in sending segmentsof different Representation sets. A typical use case is for streaming oflayered video (a layer being a view in multi-view or a scalability layerin scalable video) where segments from one layer would be interleavedwith segments with one or more other layers.

Back to FIG. 17, based on the push policy as defined in the MPD file,the server determines at step 1701 the number of segments to be pushed.This number is directly inferred from the SegmentIdx attribute value: ifthe operator is not used in the attribute value, this number is equal to1; otherwise (operator is “−” or “+”) the number is equal to the operandvalue and is assumed infinite when the operand is “*” (but limited byother constraints and by the number of existing segments).

Next, an iterative process composed of steps 1702 to 1705 is applied bystreaming server until the number of segments to push is reached (test1702) to mark each of the segments to be pushed.

For each iteration, the server retrieves at step 1703 the list ofsegments defined in the MPD file that respect the PushPolicy constraints(Adaptation Set, Representation, Period and Segment constraints andoptional conditions).

If the list of segment is empty or all its segments are already marked(test 1704) the process ends and the server starts sending (step 102above) the response to client's request.

Otherwise, the first segment of the list is marked in step 1705 as to bepushed during steps 103 (PUSH_PROMISE) and 104 (promised segments).

In these MPD-based examples of declaring the push policy, one pushpolicy is defined using a PushPolicy element (see 1600 in FIG. 16).

It is recalled here that the description file describes the media datausing a plurality of media data attribute levels, namely the Period,AdaptationSet and Representation elements defined above.

As a slight variant to the above, various shared push policies may bedefined at various respective levels of the description file. This is tobe able to define various push policies depending on the level(Adaptation Set, Representation, Period) concerned, so as to adapt thepush strategy to the content of the media stream.

This is illustrated through FIG. 23 in which the push policy is definedusing for example the “SupplementalProperty” descriptor at the desiredlevel, here at the Representation level.

Using a push policy per <MPD> level makes it possible to have a constantand same push strategy across media.

Using a push policy per <Period> level makes it possible to have a pushstrategy that can vary along time.

Using a push policy per <AdaptationSet> level makes it possible to havea media-adapted push strategy.

Using a push policy per <Representation> level makes it possible to havea push strategy that can be adapted to the media characteristics(bandwidth . . . ).

In the example of FIG. 23, the push policy specified at theRepresentation level is configured to push more segments for low bitrate video segments (2300) than for high bitrate video (2301), so toavoid using too much bandwidth with push data.

Note that the explanations above with respect to the syntax of the pushattributes may also be applied to this slight variant. In particular thepush policy can be signalled in a manifest as a new element (as in FIG.16), or using an existing descriptor with a new schemeldUri (as in FIG.23) or as a new descriptor (not represented) or any means compliant withMPD schema or MPD schema extension points.

The MPD could also contain a list of alternative PUSH policies each onehaving a unique identifier (see below for more explanation about thelist).

In other alternative embodiments, a push policy may define that segmentsfor complementary Representations are systematically pushed, for exampleusing the following syntax:

<push_policy Segments=‘+complementary’>

or value=‘complementary’ when using a DASH descriptor.

In case of layered video, this means that for a requested video segment,each segment at the same time from all Representations declared ascomplementary Representations (typically through a dependencyIdattribute in the MPD signalling dependencies between differentRepresentations) would also be pushed.

Another push policy could also consist in pushing segments fromassociated Representations, signalled either with the @associationIdattribute or with a role=‘supplementary’.

In case of fully server driven streaming, the push policy could provideinformation on whether the server behaviour has to be ‘aggressive’ (or‘optimistic’) or ‘conservative’, i.e. respectively trying to pushsegments of higher quality or trying to push at the same quality level(preserving bandwidth).

In other embodiments, the push policy is transmitted in a dedicated HTTPheader, referred to as “push-policy” header. That is the push policyinformation describing the shared push policy is embedded in a header ofan HTTP frame transmitted from the server device to the client device.

These embodiments make it possible to vary the push policy over time,since they are no longer dependent on the transmission of the MPD fileas above and the client and the server exchange using HTTP/2 protocol.

FIG. 18 is an example of communication between a server and client witha push policy transmitted in a HTTP “push-policy” header (the headername “push-policy” is just an example).

The push-policy header includes a list of push attributes, each defininga constraint on the data to be pushed. In particular, the syntax of thePushPolicy described previously may be transcribed to HTTP headersyntax.

In FIG. 18a , the server in response to a MPD request from the client(arrow 1800) transmits (step 1801) the push-policy in an HTTP headeraccompanying the MPD sent, to share the push policy.

For instance, the push policy specifies that the segment following arequested segment will be pushed. As consequence, when the clientrequests (arrow 1802) segment Data1.1, the server sends (arrow 1803)PUSH PROMISE for segment Data2.1 and then the data of segment Data1.1(arrow 1804).

Any syntax could be used to define which data is about to be transmittedfor subsequent segment request: a MPD-specific one or a more abstractone based on DOM tree node traversal.

In a specific embodiment dedicated to dynamic shared push policies, theclient may request a specific push policy, i.e. may update the sharedpush policy, for example if the current shared push policy is notadapted to its needs or may be improved.

That means that the client device sends push policy update informationembedded in a header of an HTTP frame to the server device.Correspondingly, the server device receives push policy updateinformation embedded in a header of an HTTP frame from the clientdevice. The server device may thus update accordingly the shared pushpolicy before determining non-requested media data from other media datarequested by the client device (for instance, for the next request).

In an embodiment, the push policy request from the client is conveyed inan HTTP header or request named “push-policy-request” (name here is justan example).

FIG. 18b illustrates client-server exemplary exchanges when the clientrequests a new push policy.

The beginning of the exchanges is the same as in FIG. 18 a.

After receiving segment Data2.1, the client identifies that the currentpush policy should be modified, for instance because the availablebandwidth is stable enough to let the server push more segments inresponse to a segment request.

As a consequence, the client sends at step 1805 a push-policy-requestthat asks the server to push more segments (3 instead of 1) for each newrequest.

The server positively answers this push policy request with an OK 200,at step 1806. This positive answer means the server will use the newpush-policy described in push-policy-request for any new request fromthe same client.

If the server does not want to change its push-policy, it returns anerror code answer to notify the client that the push policy request isrejected.

Next, when the client requests at step 1807 a next segment Data3.1, theserver answers at step 1808 with PUSH PROMISE for the next threesegments Data 4.1, Data5.1 and Data 6.1.

FIG. 21 illustrates, using a flowchart, steps of the process at theserver side when using the HTTP request for sharing push policy, whileFIG. 22 illustrates, using a flowchart, steps of the process at theclient side when using the HTTP request for sharing push policy.

In comparison to the process of the FIG. 14, the server includes newprocessing steps (2100 to 2105) to handle a push policy request from theclient and also to send the initial push policy and updates thereof.

If the request received by the server is a push policy request from theclient (test 2100), the server first parses the push policy request atstep 2101 in order to extract the constraints of data push proposed bythe client.

During this step, the server may decide to follow the push policyrequested by the client. In such case the server updates its internalpush policy (step 2102) and sends an OK 200 response to the client atstep 2103, in order to validate the proposed push policy.

Otherwise, when the server discards the push policy (for instancebecause the proposed policy is too costly in terms of resources orcannot be applied), step 2102 does not modify the internal push policyat the server and an error code is transmitted to the client at step2103.

According to a specific embodiment, the server may in addition updateits push policy independently of the client's requests. In such case,the server determines a push policy during step 1402 and may eitherdecide to change its characteristics (for instance by analysing therequests performed by the client and network characteristics) or seethat the determined push policy is different from the current one. Insuch a situation, the server has to share the new push policy with theclient if the latter is not already aware of it (test 2104), in whichcase the new push policy is transmitted in an HTTP header in step 2105.

The corresponding process at the client side is explained with referenceto FIG. 22. As for the server processing, new processing steps (2200 to2204) are added in comparison to the process of FIG. 14 in order toprocess push policy messages and perform push policy requests.

After having determined the current shared push policy (i.e. the pushpolicy of the server) in step 1400, the client may desire a new pushpolicy, for instance to reduce the number of HTTP requests to send forretrieving segments of a media stream. Thus, when a new push policy isrequired by the client (test 2200), the client sends at step 2201 anHTTP request with a “push-policy-request” as described previously.

The response to this request is processed in step 2204 in which theclient checks whether or not the server validates the request byreturning an OK 200 response or otherwise an error code.

If the server returns an OK 200 response, the current push policydetermined in step 1400 is replaced by the requested policy. Otherwiseit is unchanged.

In addition to the process of FIG. 14, when the client receives a framewith a new push policy from the server (test 2202), the push policy isparsed and stored (step 2203) in memory in order to be retrieved in anext occurrence of step 1400.

It has to be noted that when the push-policy request is in a frame thatalso includes other data (e.g. media data), the other data are processedthrough steps 109-111-113-115.

Whereas the above HTTP-based examples use an HTTP request to fullydefine the push policy to be applied, one specific embodiment may relyon having a set of the same predefined push policies defined at bothclient and server sides, and each having a unique identifier. In thiscase, the HTTP request is only used to specify an identifier of the pushpolicy to be used from amongst the set. This specific embodiment reducesthe size of the HTTP request.

In one embodiment, the push policy request is sent as an additionalHEADER of one of the HTTP requests used for requesting one of the serverresource: typically, the push policy request is sent in an“Accept-Push-Policy” HTTP header in the GET request for the MPD file.

In another embodiment, the client specifies several “Accept-Push-Policy”in one HTTP request to indicate the list of push policies supported (orrequired) by the client. In response to the HTTP request the server mayeither choose one of the push policy in the proposed list and thenspecify the push policy in the HTTP response or respond by a new pushpolicy if none is supported.

In yet another embodiment, the push policy request is sent in adedicated HTTP request independent of any of the resource known by theserver. For example, a GET (or POST) request is formed with an URLcorresponding to none of the resource of the web page, for instancehttp://server/push_policy and also with at least one Accept-Push-Policyheader.

In yet another specific embodiment, the set of alternative push policiesmay be defined in the MPD file exchanged between the server and theclient, each having a unique identifier. One of the push policies may bemarked as default push policy selected by the server. The client mayspecify which push policy should be used by sending a new push policyrequest which includes the identifier of the push policy to be used inreplacement of the default push policy.

In one embodiment, a specific push policy is defined to indicate whichsegment will be pushed just after the request to the MPD document forfast start.

In a hybrid approach, the push policy information describing the sharedpush policy is defined by a first push policy part and a second pushpolicy part, the first push policy part being inserted in thedescription file (MPD), and the second push policy part being embeddedin a header of an HTTP frame transmitted from the server device to theclient device.

For instance, the MPD may define the push policy with template argumentsthat are then defined (or even overloaded) by the server thanks topush-policy HTTP request. As an example, the push policy defined in theMPD file may be: <PushPolicy SegmentIdx=“parameter”/> and the value ofthe variable “parameter” may be defined in a push-policy HTTP request.In this example, the second push policy part comprises (only) one ormore values for one or more associated variables defined in the firstpush policy part.

Using the push-policy-identifier-based approach described above, thedescription file may include the description of a plurality of candidatepush policies, and the second push policy part may thus comprise anidentifier of a candidate push policy from said plurality, whichidentified candidate push policy thereby forming the first push policypart.

In another embodiment to declare the push policy to the client, the pushpolicy relies on the <Role> descriptor defined in the MPD to indicate inwhich representation the push data will be selected. Typically, the pushpolicy may specify that the push strategy will use a segment in aRepresentation with an “alternate” or “supplementary” role value.

In another embodiment, the document of resources, for example astreaming manifest or an HTML page, is transformed into a priority treethat is browsed to determine the resources to be pushed after a GETrequest is received. The navigation within the priority tree may beperformed thanks to an XPath request. In this approach, the push policyinformation includes an XPath expression to be evaluated on a treerepresentation of the document of resources to identify the secondnon-requested media data.

For example, in the streaming manifest a “following[name()“SegmentURL”][2]” XPath expression could be used to select, as segmentsto be pushed, the next two segments following the segment requested bythe client in the GET request. Also for the chapter-switching use case,a “((following[name( )“Period”]//SegmentURL)[2])” XPath expression makesit possible to select two first segments of the following Period forpreloading the first two segments of each chapter. For example, when theclient requests segment 2401 in the MPD file of FIG. 24, segments 2402and 2403 of the following Period are also transmitted by the server aspushed data.

In addition, the priority tree could be first re-ordered for exampleusing an XSLT instruction in order to simplify the XPath expressionwriting for advanced push policy rules. The XSLT instruction makes itpossible to reorganise the tree before applying a push policy. The XPathexpressions is preferably transmitted to the client, for instance in oneHTTP header and the XSLT stylesheet is defined in a web page. Thisapplies in particular to HTML documents for example to group allpictures declared in the document, all CSS resources as consecutivenodes at the same level of the DOM tree.

For instance, the tree 2501 of FIG. 25 represents an HTML page withdifferent resources of different types: hashed nodes (2511-2514)correspond to image resources and nodes in plain color (2521-2524) arescripted resources (CSS or Javascript). The tree 2502 is an example ofXSLT transformation result to group the resources by type (images in2530 and scripted resources in 2540). A simple XPath expression couldthus be defined to indicate that some resources for a given type will bepushed once the first resource of this given type is requested.

In all the embodiments described above, it is very likely that for eachclient request the server replies with several PUSH PROMISE if the pushpolicy requires several segments to be pushed.

For instance, MPD 1900 of FIG. 19 has a push policy which indicates thatthe three segments following a requested segment will be pushed (see<PushPolicy> element). Consequently, if the client requests theinitialization segment with a GET request for media 1901 with a byterange equal to 0-999, the server will send the three PUSH_PROMISEmessages 1902 during step 103.

In one embodiment, if the identified second media data comprise aplurality of media segments, each requiring an announcement message(i.e. a PUSH_PROMISE), the corresponding plurality of announcementmessages may be merged into a single announcement message to betransmitted to the client device.

To achieve this situation, as shown in FIG. 20, the processing at theserver preferably includes a pre-processing step 2000 just beforesending the push promises in step 103, compared to the general processof FIG. 14. The pre-processing step seeks to perform the above-mentionedmerger of announcement messages.

When the push promises include byte range requests as in 1902, the listof push promises 1902 is browsed to generate a reduced set of pushpromises 1903 that contains consecutive byte range addresses. Next, eachset of push promises 1902 is replaced by a reduced set of push promises1903 with a contiguous byte range equal to the concatenation of the byteranges in the push promises set or by a single push promise with a listof non-contiguous byte ranges for example 1905.

For instance, the three push promises 1902 are replaced by the singlepush promise 1903 shown in FIG. 19.

This approach of merging push promises makes it possible for the clientto cancel the send of push data in a simpler way and at lower bandwidthand processing costs. This is because the client just has to close asingle stream for the single push promise instead of closing severalstreams for each of the non-merged push promises.

In an alternative, even if the push promises have disjoint byte rangeintervals, all push promises may be replaced by a list of byte ranges(where consecutive byte range intervals have been concatenated).

In addition, if the push promises do not include byte range intervalsbut rather different SegmentURL values, the push promises may also beconcatenated to generate a single push promise message as follows: themethod of the generated push promise message is defined as MGET (formultiple GET) and the path field is a list of segment URLs asrepresented in 1904. Similar to the previous embodiment, the client hasto close the single stream corresponding to the generated push promiseto cancel the push of all segments.

Note that the server may include END_SEGMENT flags at the end of eachsegment in the data then transmitted, in order to ensure that the clientis able to parse and identify each pushed segment.

In addition, the SETTINGS frame of HTTP/2 is extended to include a newSETTINGS_ENABLE_GROUP_PUSH_PROMISE parameter that makes it possible toindicate if the grouping of push promises is allowed for the streamingsession.

Embodiments of the invention may make it possible to have a DASH faststart because one or several roundtrip(s) can be avoided. This aspect ofthe invention is now described with reference to FIGS. 26 to 28.

The DASH fast start feature may be used with any communication approachas described above with reference to all or part of FIGS. 7 to 12 and 14to 25.

FIG. 26 shows exemplary methods respectively implemented by a server andby a client device in accordance with the teachings of the invention, inorder to obtain a DASH fast start.

As in the standard process just described, the first step consists forthe client to request the description file, here an MPD file (step2650). The client then waits for the server's response (step 2651).

In the meantime, the server analyses the MPD file (step 2652), inparticular to identify (step 2653) initialization data which will helpthe client to start faster, as explained below. An exemplary embodimentfor step 2653 is described below with reference to FIG. 27.

Once initialization data is identified by the server, it sends aPUSH_PROMISE frame to the client at step 2654 to indicate its intentionto push initialization data without waiting for a client's request.

Possibly, it signals in addition that it will also push initial mediadata (step 2656) by sending another PUSH_PROMISE frame including headerfields that allow the client to identify the concerned resource, i.e.the concerned initial media data, such as :scheme, :host, and :path.

Both in the case of a PUSH_PROMISE frame for initialization data and ofa PUSH_PROMISE frame for initial media data, other header fields arealso added by the server to indicate how much the server is confident inthe data it has decided to push: in the present embodiment, aconfidence_level parameter is associated to (i.e. included in a headerof) the PUSH_PROMISE frame. The determination of the confidence_levelparameter is described below with reference to FIG. 27. The server canalso insert a specific DASH header to unambiguously indicate the segmentthat it intends to push.

To minimize the risk that a client will make a request forinitialization data and first media data that are to be pushed, thePUSH_PROMISE frames should be sent prior to any content in the response,i.e. step 2654 and step 2656 should occur before a step 2655 of sendingthe MPD file from the server to the client device.

Thus, when PUSH_PROMISE frames are sent to the client device, the serversends the MPD file to the client device at step 2655.

If the server has not received any CANCEL or ERROR message from theclient device in the meantime, it starts pushing initialization data(step 2657) and first media data (step 2658).

PUSH_PROMISE frames and pushing of data from the server to the clientdevice is for instance performed in accordance with correspondingfeatures being developed in the frame of HTTP 2.0, as described forinstance in the document “Hypertext Transfer Protocol version 2.0,draft-ietf-httpbis-http2-latest”, HTTPbis Working Group, Internet-Draft,Jun. 24, 2013 (available for instance athttp://http2.github.io/http2-spec/).

Upon receipt at the client device, the initialization data can be usedby the client to set up the decoder(s) (step 2659) and the first mediadata are buffered (step 2660) until a sufficient amount of data isavailable for decoding and rendering (e.g. displaying) without displayfreeze.

When the client has fully received the MPD file, it parses it (step2662) and starts decoding and displaying (step 2663) provided enoughdata are buffered (step 2661). If this is not the case, and the clientdevice knows from PUSH_PROMISE frames sent by the server (see step 2656)that more segments will be sent, it waits at step 2664 for thecompletion of the push of first media data from the server. During thisidle step 2664, the client device may prepare the next requests forsubsequent segments that will be issued in a standard client controlledDASH (step 2665), as already explained above. This is possible becausethe client device has received information on the initial media data tobe pushed (or being pushed) in the corresponding PUSH_PROMISE frame (seestep 2656 above) and can thus prepare requests for the temporal segmentimmediately following the last temporal segment intended to be pushed bythe server.

The client device, when it has fully received the MPD, may also use theinformation on initial media data received at step 2656 to check whetherthis initial media data fills the buffer and, if not, to send a requestfor the following media data (e.g. media data corresponding to atemporal segment following the temporal segment represented by initialmedia data) according to the standard client controlled DASH processprior to step 2661 (contrary to what is shown in FIG. 26 which shows acase where the pushed initial media data fills the buffer). This enablesthe client to correct a bad estimation from the server on the quantityof first media data to push.

This process enables a streaming client to start displaying mediaearlier than in standard manifest-based streaming. Indeed, the startupdelay is reduced because the number of HTTP roundtrips on the network isreduced to get initialization data and/or initial media data.

This process remains however compliant with the current DASH standard,because:

-   -   there is no modification of the MPD file: its transmission        remains light and fast;    -   the behaviour of standard DASH clients (i.e. not benefiting from        the teachings of the invention) may be unchanged: such client        devices would ignore unrecognized HTTP headers and, when not        accepting the push feature, would simply have to perform more        requests/responses and thus spend more time to start the        presentation.

FIG. 27 describes an exemplary method implemented at the server sidefollowing a request for the manifest (or description file) from a clientdevice.

This method seeks to identify the most relevant initial data to push inadvance so that the client can start rapidly the display of the mediapresentation.

In step 2700, the request for the manifest is received. The server thenchecks at step 2701 whether the client device inserted some preferencesin the request. This may be done via a dedicated HTTP header like forexample to express a transmission rate for the media presentation and apreferred language for audio stream:

GET http://myserver.com/presentation/pres1.mpd \r\n

Prefered-MediaRange: bw=2000;lang=FR\r\n\r\n

If the request includes preferences (test 2701 true), the serveranalyses the client's preferences (step 2703) and sets itsconfidence_level parameter to the value “high” (step 2704).

If no indication is provided in the request (test 2701 false), theserver checks at step 2702 whether it has already registered serviceusage information (logs) for this client (i.e. statistics or usage databased on prior exchanges between the user or client device and theserver) or the information from the User-Agent header. Indeed, theUser-Agent header is defined as a HTTP header in RFC2616 (see e.g.http://www.ietf.org/rfc/rfc2616.txt) and provides a means forapplications to exchange information like for example operating system,browser type, application name, etc.). For instance, the DASH server mayhave an authentication scheme for the clients before being able to usethe service; in a variation, it can be a user logging before gettingaccess to the service. With such means, the server can link mediaparameters to a connected user or device.

When prior usage information (logs) is available for the concernedclient device or user (test 2702 true), by parsing the logs at step2705, the server can deduce most frequent usages for a given client oruser. For example, it can deduce that the user or client device alwaysselects an audio stream with French language and video stream in HD(High Definition). Moreover, the server can know whether this is a firstrequest in the open TCP connection or not (client connected to serviceand requesting a second media presentation). In this case, the bandwidthestimation can be more accurate and reliable and the TCP congestionwindow may be bigger than for a first request. This can impact thechoice made by the server in terms of suitable Representation,

By registering DASH quality metrics, the server can have in its logs thechanges among various representations the user/client usually performs.From this, the server determines the usual behaviour between“aggressive” or constant depending on the frequency of changes (bychanges we mean switches to other Representation, whatever thecriterion: bandwidth, resolution, frame rate, etc.). An aggressiveclient is a DASH client that will automatically switch to a differentrepresentation when its context changes. As an example, when monitoringbandwidth or buffer occupancy, an aggressive client will request aRepresentation with a different bandwidth as soon as a newRepresentation has characteristics closer to the client's contextcompared to the current Representation. In opposition, a constant clientwill try to avoid frequent Representation switches in order to maintainstable quality and display rate. When the user/client device behaviouris rather aggressive in terms of adaptation, the server then knows thatwhatever it selects as initial representation to start the streaming,the client will try to adapt in the following first seconds or minutesof streaming.

When preferences are deduced from logs, the server sets itsconfidence_level parameter to the value “mid” at step 2706. Indeed, thisinformation may be a bit less relevant than explicit preferencessignaling by the client itself (test 2701 true).

When no log information is available (test 2702 false), then the serverputs its confidence_level parameter to the lowest value: “low” at step2707. This indicates that the server is performing a best guess on theinformation it pushes because it has no a priori information to decide.Further process in this case is described below (see step 2711).

In parallel of this confidence_level parameter computation, the servermay parse the manifest at step 2708. In cases where the manifest is notliable to change very often (especially for on-demand service, inopposite to live service), the parsing of the manifest can be performedoffline, once for all, by registering the description of the variousRepresentations in a lookup table. This lookup table may also be used bythe server to link clients' logs to some parts of the mediapresentation. This enables a faster log processing (see step 2705described above) to deduce some client's preferences.

The parsing of the manifest (step 2708) provides information to theserver at the time of selecting (at step 2709) a suitable Representationas initial Representation (i.e. initial media data) to start thestreaming.

Both steps 2703 and 2705 (obtaining preferences respectively in therequest or based on usage data from prior exchanges) consist intranslating preferences or usages from client device/user into concreteparameters that would match MPD attributes. For example, it can bebandwidth, width and height of the video, the kind of codec in use, thelanguage for subtitles or audio streams. Then, from the obtained valuesfor these parameters, the server compares with values in the manifest toidentify at step 2709 the most convenient Representation to push to theclient.

It may be noted that this step 2709 is typically what the client deviceperforms continuously in a dynamic and adaptive streaming protocol likeDASH. Here, the same step is performed by the server at the beginning ofthe streaming session by MPD parsing means.

In case no suitable Representation can be deduced in 2709, test 2710 isfalse and the server puts its confidence_level parameter to the “low”value (in step 2707 previously mentioned).

When the confidence_value parameter has the “low” value (either becauseno preferences could be determined or because no suitable Representationcan be found based on preferences), the server decides at step 2711 toselect the simplest Representation. For video, for instance, thesimplest Representation may be the Representation with lowest spatialresolution and designed for lowest bandwidth.

According to a possible complementary feature (not represented in FIG.27), when there is no ambiguity on the codec (i.e. all videoRepresentations have the same value for the codec attribute, i.e. thesame codec, for example HEVC, has been used to encode all the videoRepresentations), the confidence_level parameter may be raised to thevalue “mid”.

The next step after step 2711, or when a suitable Representation hasbeen found (test 2710 true), consists in identifying the initializationdata (step 2712). Indeed, in the DASH manifest (or description file),initialization information can be signaled in different ways: it can beexplicitly put in an Initialization element of a SegmentBase,SegmentList or Segment Template element that provides a direct URL tothe initialization data.

In this case, this URL is put in a header field of the PUSH_PROMISEframe (see step 2654 described above with reference to FIG. 26) thatwill allow the client to identify the resource promised to be pushed (byspecifying the variables :scheme, :host, and :path and eventually:Range).

When initialization data is not explicitly described, this means thatmedia segments are self-initialized. In such case, the server has toparse the beginning of the segment (e.g. segment index information boxesfor segments in mp4 format). Based on this analysis, it can build thecorresponding URL with the appropriate byte range that will be put asheader in the PUSH_PROMISE frame.

Once identified, a PUSH_PROMISE frame for initialization data isimmediately sent to the client (step 2713, corresponding to step 2654 inFIG. 26), immediately followed here by the push of the initializationdata (step 2717 a, corresponding to step 2657 in FIG. 26). Wheninitialization data are received, the client can then initialize itsmedia decoders (step 2717 b).

Optionally, to improve the segment signaling and later identification bythe client device when processing the PUSH_PROMISE frame (see step 2806described below), the server can indicate in step 2713: the nature ofthe pushed data: initialization or media or both (in case ofself-initializing segments); the parameters of the URL template or anindication of the segment as a path in the MPD representation tree ofFIG. 5b (for example: P2AS21R211S1; i.e a concatenation of element typefollowed by an identifier). It may be noted that this requires theclient device to have received the MPD. Then, the server can decide toadd this specific information only in the PUSH_PROMISE messages that itthinks will be processed after MPD reception by the client device. Tohelp the decision at the client device on accepting or not aPUSH_PROMISE before the MPD reception and parsing, the server canindicate, instead of the segment path in the MPD, qualitativeinformation on the pushed segment, such as whether it is a segment froma base layer or an enhancement layer; according to another example, theserver can place in a header the attributes of the selectedRepresentation with their values.

According to a possible embodiment (not represented on FIG. 27), whenparsing the manifest at step 2708 determines that initialization data ispresent in top level elements of the manifest (i.e. whatever theRepresentations, the initialization data is common to allrepresentations; for example in case of dependent Representation), theserver can immediately (i.e. concurrently with step 2708) send thePUSH_PROMISE frame designating initialization data with aconfidence_level parameter set to the value “high” since there is norisk of mismatch between pushed data and what the client would havechosen. The benefit of sending the confidence_level parameter with thePUSH_PROMISE frame, for example as a HTTP header, is that it can helpthe client device in accepting or cancelling the push promise (see thedescription of FIG. 28 below).

Thanks to this feature, the client will receive even earlier theinitialization data required to setup its decoders (as the PUSH_PROMISEframe is sent early). This also works when initialization data is uniquefor a given media type (e.g. one single InitializationSegment perAdaptationSet whatever the number of Representations in thisAdaptationSet). This even faster push would come just after the parsingof the manifest (step 2708 described above), thus before processing logsor preferences (steps 2701, 2703 and 2705 described above).

Then, if the confidence_level parameter previously determined by theserver is greater than or equal to the “mid” value (test 2714), theserver takes the initiative of pushing the first media data it considersas suitable for the client.

This is done iteratively in two steps: first a PUSH_PROMISE frame issent (step 2715, corresponding to step 2656 in FIG. 26) and then thepush of first media data starts in step 2719. This is repeated for eachfirst media data segment that has been selected to be pushed in step2709.

According to a possible embodiment, when consecutive media segments arepromised to be pushed (i.e. a plurality of PUSH_PROMISE are sent forrespective media segments), the PUSH_PROMISE associated to the currentmedia segment is marked as a child or a follower of a previousPUSH_PROMISE (step 2716). This can be put as a new HTTP header in thePUSH_PROMISE frame if the server is stateless or kept in a table if theserver is stateful. Keeping this relationship can be useful to performhierarchical cancel on push promises (as described below with referenceto FIG. 28).

A possible schedule of the various transmissions of data is as follows:before actually pushing first media data, the server starts pushing theinitialization data in step 2717 a mentioned above; in parallel tosending the PUSH_PROMISE frame relating to first media data andinitialization data, the server also sends the MPD file (manifest) atstep 2718 and keeps the stream open until the pushed data are completelysent.

In another embodiment, test 2714 can be avoided to push first media datawhatever the level of confidence. But in case the confidence_levelparameter is set to “low”, the server may wait for a potential CANCELfrom the client before actually pushing the first (or initial) mediadata.

When pushing the first media data, the server determines the overallquantity of data to push and the speed to use (flow control).

Regarding the first aspect, the server can exploit information from themanifest such as for example the minBufferTime attribute mentioned atthe beginning of the manifest. Using this attribute, and considering theRepresentation selected in step 2709 or 2711, and given the segmentduration attribute also provided in the manifest, the server easilydetermines the number of segments to push to fulfill the minBufferTimeconstraint (i.e. the quantity of segments, hence the quantity of data,forming the initial media data to be pushed). Advantageously, whenparsing of the manifest (step 2708) is performed offline, this number offirst media segments can be recorded in a table in a memory of theserver.

Regarding the second aspect, given the duration of the segment and thebandwidth of the chosen Representation, an estimate of the requiredbitrate may be obtained by the server. This provides, mainly for videosegments, the transmission rate to use. For example for a compressedvideo representation with bandwidth equal to 1.6 Mbits/s having segmentsof 5 seconds duration, each segment would represent 1 mega-byte of datato send. By default, the flow control in HTTP v2.0 provides a streamwindow size at most equal to 65535 bytes. Thus, in our example, thismeans that the client would have to send back to the server anacknowledgement for each packet of 65536 pushed bytes, so in our examplemore than 15 times per segment! Since we aim at reducing networkroundtrips and traffic when using the push feature under developmentHTTP 2.0, we see clearly that there is a need here to modify the defaultbehaviour (actually the default congestion window size) to enable DASHfast start (by reducing network traffic).

In case the client device sends preferences included in its request forthe manifest, it can also indicate that a SETTINGS frame is to be sentimmediately after the request; this SETTINGS frame specifies forinstance an initial window size (SETTINGS_INITIAL_WINDOW_SIZE) in linewith its buffering capacities. According to a possible variation, thisSETTINGS frame can be sent at connection setup time. Another possibilityis for the client device, when acknowledging the first pushed data, tosend a WINDOW_UPDATE with appropriate size.

FIG. 28 describes a possible method implemented by the client device,when exchanging data with the server executing a method for instance asdescribed in FIG. 27, in accordance with the teachings of the invention.

According to a possible application of this method, the client deviceconnects to the server in order to benefit from a video on demandservice. The connection establishment between the client and the serveris conventional. In the present example, both the client device and theserver are able to exchange messages using HTTP/2.0 protocol describedfor instance in the already mentioned document “Hypertext TransferProtocol version 2.0, draft-ietf-httpbis-http2-latest”.

At a time (for instance when the user at the client device selects agiven video), the client device gets information from the server on theaddress (e.g. the URL) of a manifest describing a media presentation(here the video the user would like to see).

The client device then prepares a request to download the manifest (step2800). In a preferred embodiment, the client adds through HTTP headerssome preferences on the video resolution, codecs, bandwidth it supports(step 2801). The client device then sends its request to the server(step 2802).

In the present embodiment, the client device then sends at step 2803 anHTTP/2.0 SETTINGS frame to indicate an initial window size(SETTINGS_INITIAL_WINDOW_SIZE) in line with its buffering capacities(see the document “Hypertext Transfer Protocol version 2.0,draft-ietf-httpbis-http2-latest” mentioned above, section 3.8.5).

In step 2804, the client device starts processing the various serverresponses: receiving data forming the manifest and parsing it (step2805) but also the PUSH_PROMISE frame(s) sent by the server (step 2806).

Before deciding to accept or to cancel the push(es) designated in thePUSH_PROMISE frame(s), the client builds the URL of the resource theserver intends to push (step 2806) and checks (step 2807) theconfidence_level parameter that has been included in the PUSH_PROMISEframe by the server.

In parallel and when the manifest (or description file) is fullyreceived, the client device builds (step 2808) the list of desired mediasegments it would like to get (i.e. the list of versions of each segmentthat best suit its needs) and initializes a current segment_indexvariable to 0 (step 2809). The first step in processing the PUSH_PROMISEconsists (step 2810 a) in checking the confidence_level parameter. Then,depending on (predefined) client settings or user preferences the clientmay decide to reject the PUSH_PROMISE under a certain level ofconfidence, for example the PUSH_PROMISEs for which the PUSH_PROMISEframes include a confidence_level parameter with a “low” value.

If the client can match (step 2810 b) the URL mentioned in thePUSH_PROMISE frame with the URL of a desired segment (as derived fromthe manifest in step 2808 as just mentioned), it initializes a table fora list of pending segments being transmitted with their transmissionstatus (step 2811). If the client cannot identify the segment intendedto be pushed by the server in step 2810 b in the list of desired mediasegments, it then cancels the push (step 2812) by sending an appropriateCANCEL instruction to the server.

To facilitate the segment identification at step 2810 b, the client canexploit additional header information like for example the index of thepushed segment, as the path in the MPD tree representation (see FIG. 5b), or the URL template parameters when the description file (i.e. theMPD file or manifest) relies on SegmentTemplate.

This is a specific CANCEL message here (step 2812) since using thehierarchical relationship inserted by the server when building thePUSH_PROMISE (see the description of FIG. 27 above), the client can senda recursive CANCEL that will result in the cancellation of the currentPUSH_PROMISE plus the following ones.

According to a possible embodiment, when the client device cannotinterpret the push promise, it stops by default all pushes of media datacorresponding to the next temporal segments of a media resource.

This new usage of the CANCEL instructions will avoid the client torepeat CANCEL messages once it is desynchronized with the server interms of media segment identification. In such case, the client willfall back to a pull mode.

When the segment to be received by the push from the server correspondsto a desired segment (test 2810 b true), the client then continues theprocessing of the PUSH_PROMISE frames (test 2813 and loop on step 2806).

When all PUSH_PROMISE frames have been processed, the client deviceexpects and begins receiving and buffering (step 2814) datacorresponding to the accepted PUSH_PROMISE.

When enough media segments are received in the reception buffer of theclient (test 2815), they are processed by the client (2816). The currentsegment_index variable is then updated with the ordering number of thefirst segment in the list (step 2817). It should be noted that not allclients may get access to the client's buffer. For example, webapplications in particular do not usually have access to the web browsercache. In such a case, the server may send the list of pushed segmentsto the web application client directly. This information may beexchanged from the server to the client using a web socket connection,for instance.

When all pushed media segments have been processed, the client can thengo back to standard pull-based DASH (step 2818), starting requestingdata corresponding to the next segment, designated by the variablesegment_index+1. In parallel, the pushed segment data are used to startthe decoding and the display of the selected video.

FIG. 13 is a schematic illustration of a device according toembodiments. The device may be a server, a client or a proxy. The devicecomprises a RAM memory 1302 which may be used as a working memory for acontrol unit 1301 configured for implementing a method according toembodiments. For example, the control unit may be configured to executeinstructions of a computer program loaded from a ROM memory 1303. Theprogram may also be loaded from a hard drive 1306. For example, thecomputer program is designed based on the flowcharts of FIGS. 8-12, 14,15, 17, 20-22 and 26-28, and the above description.

The device also comprises a network interface 1304 which may be a singlenetwork interface, or comprise a set of network interfaces (for instanceseveral wireless interfaces, or several types of wired or wirelessinterfaces). The device may comprise a user interface 1305 fordisplaying information to a user and for receiving inputs from the user.

The device may also comprise an input/output module 1307 for receivingand/or sending data from/to external devices.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive, theinvention being not restricted to the disclosed embodiment. Othervariations to the disclosed embodiment can be understood and effected bythose skilled in the art in practicing the claimed invention, from astudy of the drawings, the disclosure and the appended claims.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. A single processor or other unit may fulfill the functions ofseveral items recited in the claims. The mere fact that differentfeatures are recited in mutually different dependent claims does notindicate that a combination of these features cannot be advantageouslyused. Any reference signs in the claims should not be construed aslimiting the scope of the invention.

The invention claimed is:
 1. A method of providing media segments by aserver device to a client device, the method comprising the followingsteps: receiving, from the client device, a request relating to a firstmedia segment, transmitting to the said client device the first mediasegment in response to the request, identifying, using a push strategyshared with the client device, one or more second media segments to bepushed to the client device which transmitted the request relating tothe first media segment, sending, to the client device, a notificationindicating that the server device intends to push the media segmentsaccording to the push strategy, and pushing the identified one or moresecond media segments to the client device after sending thenotification, wherein a number of the one or more second media segmentsare identified by the push strategy shared with the client device. 2.The method of claim 1, wherein the push strategy shared with the clientdevice defines a pushing order of the identified one or more secondmedia segments to be pushed by the server device, and the method furthercomprises the step of transmitting ordering information related to thepushing order, the ordering information enabling the client device todetermine the pushing order defined by the server device.
 3. The methodaccording to claim 1, wherein the shared push strategy is identified bythe server device based on the request from the client device.
 4. Themethod according to claim 1, wherein data is transmitted to the clientdevice in the transmitting step is the first media segment.
 5. A methodof receiving by a client device, media segments provided by a serverdevice, the method comprising the following steps: transmitting, to theserver device, a request relating to a first media segment, receivingfrom the server device, in response to the request related to the firstmedia segment, the first media segment, identifying, based on anotification from the server device with which a push strategy isshared, one or more second media segments to be pushed to the clientdevice by the server device which received the request relating to thefirst media segment from the client device, and receiving, from theserver device, the identified one or more second media segments to bepushed, wherein a number of the one or more second media segments areidentified to be pushed by the push strategy shared with the serverdevice.
 6. The method of claim 5, wherein the shared push strategy isused to determine the identified one or more second media segments to bepushed by the server device to the client device.
 7. The method of claim6, further comprising receiving, from the server device, push strategyinformation describing the shared push strategy.
 8. The method of claim7, wherein the push strategy information describing the shared pushstrategy is embedded in a header of an HTTP frame transmitted from theserver device to the client device.
 9. The method of claim 7, whereinthe push strategy information describing the shared push strategy isembedded in a header of an HTTP request sent from the client device tothe server device.
 10. The method of claim 6, wherein the one or moresecond media segments are identified to be pushed in the identifyingstep based on both of the push strategy and the first media segmentrequested.
 11. The method of claim 5, wherein the push strategy sharedwith the server device defines a pushing order of the identified one ormore second media segments to be pushed by the server device; and themethod further comprises the following steps: receiving from the serverdevice, in response to the request, the notification respectivelyidentifying the one or more second media segments to be pushed to theclient device, and receiving information related to the pushing order ofthe identified one or more second media segments to be pushed with thenotification, the information enabling the client device to determinethe pushing order of the identified one or more second media segments tobe pushed defined by the server device.
 12. The method according toclaim 5, wherein the push strategy is identified by the client device,and the request relating to the identified push strategy is transmittedto the server device for sharing.
 13. The method according to claim 5,wherein data that is received from the server device in the receivingstep is the first media segment.
 14. A server device for providing mediasegments to a client device, the server device comprising: a hardwareprocessor; and a memory storing one or more programs configured to beexecuted by the hardware processor, the one or more programs includinginstructions for: receiving, from the client device, a request relatingto a first media segment, transmitting the first media segment to theclient device in response to the request, identifying, by using a pushstrategy shared with the client device, one or more second mediasegments to be pushed to the client device which transmitted the requestrelating to the first media segment, sending, to the client device, anotification indicating that the server device intends to push the mediasegments according to the push strategy, and pushing the identified oneor more second media segments to the client device after sending thenotification, wherein a number of the one or more second media segmentsare identified to be pushed by the push strategy shared with the clientdevice.
 15. A non-transitory information storage medium readable by acomputer or a processor storing instructions of a computer program, forimplementing a method according to claim 1, when the computer program isloaded and executed by the computer or the processor.
 16. Anon-transitory information storage medium readably by a computer or aprocessor storing instructions of a computer program, for implementingthe method according to claim 5, when the computer program is loaded andexecuted by the computer or the processor.
 17. A method of providingmedia segments by a server device to a client device, the methodcomprising the following steps: receiving, from the client device, arequest relating to a first media segment, transmitting to the clientdevice the first media segment in response to the request, identifying,using a push strategy shared with the client device, one or more secondmedia segments to be pushed to the client device which transmitted therequest relating to the first media data, sending, to the client device,a notification indicating that the server device intends to follow thepush strategy, and pushing the identified one or more second mediasegments to be pushed to the client device after sending thenotification, wherein a number of the one or more second media segmentsare identified to be pushed by the push strategy shared with the clientdevice.
 18. The method according to claim 17, wherein the server devicereceives from the client device the request identifying the pushstrategy, and the server device sends, in response to the request fromthe client device, the notification to the client device.